I may as well respond... section by section
Yes, Bandana Dee is a basic character, and that's a bit of an issue. As a former fan of his, part of the reason I lost interest in him was because he never evolved past his Return to Dreamland phase. He could be a great mediator in-between Kirby and Dedede's rivalry, but he's not, he's a Kirby companion and little more. Concurrently, for years I've been disgruntled by modern Kirby's cast choices because they've avoided using certain characters at fitting intervals. There's no true excuse for Adeleine's absence in Triple Deluxe and Robobot, or Gooey's in Rainbow Curse and Battle Royale. In both cases, the role they could've easily served was used by Bandana Dee.
He has evolved a bit since RTDL, mainly through spin-offs, given more character through his new ability to talk, more expressions, and relations with other characters. As for being a mediator, well I think that comes down to the series evolving rather than a fault of BD himself. King Dedede has gradually been made to be an ally to Kirby, which feels like a huge improvement to what could have been a stale Mario/Bowser type rivalry. Plus, he does act as a mediator as Battle Royal has him trying to work his way up to be the best to the King, while he still values his friendship and forms team ups with Kirby.
Honestly, I think there is a good excuse for absenses of previous characters, and it just comes down to starting over. Being completely honest, the Kirby team clearly fell apart with the absence of Sakurai, leading to a very rough decade at least. Kirby wasn’t doing well and was a series that kind of took more steps backwards than forwards. Now before I speak, yes 64 is a good game, but it clearly took steps backwards from Super Star, being a lot slower and copy abilities lost the full range move set established in Sakurai’s last title. Dream Land 3, 64, and onward all tried to be Super Star but lacked fundamental parts to make it happen, then with the later GC and Wii and DS era Kirby became a spin-off series. RTDL was Kirby’s Return to Super Star, it was meant to be basic to set the ground work for the next titles and get the series back on track. I think of RTDL as a reboot, which means that they don’t want previously established characters that aren’t main to take up the spot light as they have to set ground work before diving into everything.
This leads into your next point
Whether or not Bandana Dee is truly flexible as a character is... debatable, but let's assume he is. Okay, but are you sure Adeleine and Gooey can't be? Both have never truly been given backstory despite implications, and both can endure tough climates and years of irrelevancy. Adeleine's human design honestly makes her an interesting contrast to the main trio, and Gooey's goofy nature clashes with his dark source... in a good way.
This is not true, at all...
Adeleine doesn't need Ribbon at all. I mean, Adeleine & Ribbon as a duo are cool, they work well and mixing paint summoning with crystal magic is interesting (I wish they expanded a bit more on the crystal side in Star Allies). But at the end, Adeleine would be ok alone as well.
She can't fly? She could get that heavy multi-jump that many helpers have in Superstar (such as, for example, Bonkers, who can't fly as well). Everything else can be fixed by giving her new moves. Make her able to draw a rainbow path in the sky similar to how you do in Canvas/Rainbow Curse, for example.
There are no limits in what you could do with her moveset, she can materialize wathever the developers want to, due to her skills.
And even if Ribbon has to be her partner, there isn't any need of explanation for her to be there. They are friends, that's all what it matters.
What's the reason for Metaknight to stick around nowadays? He's just there, there is no need to explain why,
I put these two together as I feel I have a similar response.
The problem with Adeline and Gooey, along with others is that they are not entry characters. To compare it to the game this site is based on, let’s look at Smash fighters. If I were to hand my controller over to a new player, would I tell them to play as Kirby or Bayonetta? Adeline and Gooey are very different, a Bayonetta of sorts. Their powers are strange and unlike anything in the game, which is very apparent in Star Allies as they have huge attacks that aren’t comparable to anyone. BD on the other hand is a spear copy ability stand in, in the same way Meta Knight and King Dedede are pretty much just basic sword and hammer. He’s an easy to grasp character with little flair, and one that you can get the hang of right away or after using the Spear copy ability. In a sense, BD in all areas is flexible, an easy 4th character that fits the puzzle. His design is as basic as the original trio, he is given just as much backstory and goals as the trio, and he plays easily so anyone can easily grasp how to play a Kirby game, which has been the main goal of having the 4 playable characters.
Adeline, as said by
Iko MattOrr
, has too much. It creates a divide as a 4th character as she is widely different, or it creates a divide between fans. Either Adeline has too much uniqueness that makes her difficult for new players to fully grasp and creates a divide in play style compared to the trio, or she is toned down to the point where she has wasted potential. Now remember, Kirby RTDL was pretty much a revival of Kirby after a ten year hiatus, so many people playing the title were probably completely new to the franchise, so throwing a curveball like Adeline or Gooey that don’t match up with the rest of the game could turn people away. Star Allies avoided this by being a 4th installment and having her as DLC after the game dropped, so new players would get the hang of everything before playing around with wild characters such as Adeline, Marx, or Susie.
Another problem is the pre-establishment of these characters. Again, RTDL was a revival, the Kirby team was clearly going a new direction and they want freedom. When characters are as popular as Adeline, than you cannot have freedom. BD was the teams baby, their experimental character that caught on. He was a character to essentially be their mark on the franchise and they can work on him as they go as they have all freedom to do so. In Super Star Ultra, he was given a backstory, and in Battle Royal they fleshed out his character more showing fear to warp stars and the true extent of his loyalty. He is a character in progress, but he is the modern mark on a now up and coming big franchise, giving the new creators something to play around with without fan backlash.
To complete my ramblings,
Quillion
, BD is a character the fan base likes. He is not over exposed, or really pushed by HAL, but he is Kirby’s 4th member. You may call him a creator’s pet, but he isn’t. He is the new of the Kirby franchise, the thing to show that Kirby hasn’t completely devolved into New Super Mario Bros. only relying on Nostalgia and characters people know, otherwise all of the fan favorites would come back in every title and only get more and more stale.
Honestly he is and has been a welcome addition that HAL clearly cares for. He may not be the most interesting, but he was never meant to be. He was supposed to be a gateway to those characters, a character that teaches you the basics of the franchise. He is simply like a Chrom, generic but is iconic and eye catching from the start, leading new players into the world of FE with a general impression of what gameplay and the personality of a lord are like. These characters get people to place interest in a franchise, and sometimes, being the gateway to having a bigger fandom may be more important than being the most interesting of the bunch. BD is everything the Kirby franchise needed at the time, an iconic new blood that shakes up the dynamic of the trio without feeling too distant in characterization or design. Basic enough for anyone to grasp, and flexible enough to change as the team tries new things.
