Literally half the Assist Trophies in the game are more worth putting effort into developing a playable character than
. Any argument that "this character didn't get in because they didn't have time and he/she wasn't important enough and they can't put every character in" died with the Plant.
If they had time to develop a Plant, they damn well had time to develop any one of Waluigi, Isaac, Bandanna Dee, Shadow, Krystal, Shovel Knight, Dixie Kong, Impa, Toad/Toadette, Lyn, Skull Kid, Midna, Ashley, Bomberman, Octolings, Tails, Chun-Li, Elma, Takamaru, Geno, Shantae, a Rhythm Heaven rep, etc. Every single one of those characters would have been a better investment in that they all had someone asking for them.
They didn't have time though and they weren't important enough. Piranha Plant got the priority because Sakurai wanted to do something different and fun with a unique concept that could appeal to some people (It does). People can complain, but at the end of the day, it doesn't change anything.
At absolute best, you get one slot back. So, pick one from that list. Got them? Good, because the rest still aren't in and those fans still aren't happy. It didn't matter if it was Isaac, Geno, Waluigi, whoever. The end result does not change even if Sakurai included another more requested fighter because almost all the hatred thrown at Piranha Plant is just every fan base looking for a scape-goat and having one that's convenient because they aren't conventionally popular.
If it wasn't Plant, it would have been Incineroar, if it wasn't Incineroar, it would have been Isabelle, if it wasn't one of those three then you'd just have endless in fighting over which one of any number of more popular characters got in. Hell, some people do whine about all three of those options for "not being ballot picks" as is and come off as excessively ridiculous for it.
Not every character "has to be asked for" and people can find new things they love through characters. Guess what? I love Piranha Plant in Smash, and I'm not the only one. I'm not trading him for one of any more conventionally popular picks personally, and that's exactly the point. Sakurai made a character to appeal to different people. The opportunity cost of Plant is the same as an opportunity cost as any other character in the regard that some people like them, others don't. You can try and spin this as an utilitarian thing... but Smash has a pretty extensive history of ignoring the utilitarian character choice in favor of something more fun or indirectly important to Nintendo/Sakurai. I mean, for god's sake, we got Ice Climbers and Mr. Game & Watch before Wario, Diddy Kong, Ridley, etc. and ROB beat how many characters to Smash and he's what, less than an enemy since he's just a toy that functioned poorly?
They're just items to be used. No choice given, no option to fight along side your favorite character, just randomly summon and prey you didn't a character like Skull Kid and Nightmare who actively hinder you during battle. Then when your Assist Trophy gets badly damage you have the option to kill them so your opponent doesn't score a point. Massive amounts of respect.
Assist Trophies are insignificant. If you get rid of one, five, ten, or even thirty nothing about the game changes. Get rid of a playable character is significant, nothing is loss when an Assist Trophy is gone. Hell, if Assist Trophies are so great and so important why are many missing from Brawl and Smash 4?
I'd miss Assist Trophies. They'd be a loss to me, and then you would actually lose a whole lot of the crossover stuff from Smash. You'd lose Shovel Knight, the biggest part of Golden Sun, Fatal Frame, Virtua Fighter, Bomberman, Dillion, ARMS, etc. You actually do lose a lot of value when you remove assist trophies (and since I know you're next step is to complain about Mii fighter costumes, we can't count any of that either, and I'm presuming Spirits go as well for all non-playable fighters). You're actively campaigning to cut down the crossover, to cut down the rich details of Smash through all of its worlds and aspects that people can recognize and enjoy, and cutting entire things out of Smash out of spite at this point. Cutting 59 Assist Trophies doesn't just casually get you Bomberman, Shadow, Isaac, or whoever the hell else. And based upon your comments it seems like you just want to reduce Smash down to a super boring character select screen focused affair with stages and little else.
Seriously, this is the video game equivalent of not liking broccoli and campaigning to have it eradicated because
you don't personally like it and think the farm land can be used for better crops.