Dr. Mario 64 literally came out 6 months prior to Melee, and the Gamecube launched the same month as Melee''s .. trust me, no one cared about Dr. Mario 64.. I doubt Sakurai threw in Dr. Mario due to his Nintendo 64 game.
Megamans situation is not portrayed poorly Megaman was nowhere near his star status that he was in the 90s.. the few games he received before Smash Wii U which the last one was 4 years prior were under the radar and barley anyone talked about them....this is mostly due to capcoms negligence of the character. Sure Megaman was sort of around but Capcom wasn't marketing him nearly as much as they used to and he was practically a relic. His Smash 4 appearance has aided his resurgence greatly.
The fact that most of those characters are from the NES days is moot because it proves the point that relevancy is none factor to Sakurai. If he cared about relevance we wouldn't have any NES era characters at all. The point is, he dosent care about it.
Also I would be hard pressed to say that a lot of these characters are from franchises no one remember's, maybe you don't because perhaps your too young, but I remember all of them except admittedly Game & Watch who pre dates my gaming years.. I had an NES and owned many of the games they come from as do many other's who play Smash. I've never heard Sakurai say he adds older characters for "shock value" seems more to me that he adds them like any other character for fans of those characters.. again because relevancy isnt as big a deal as the community likes to make it seem.
Those two were still relevant though. Dr. Mario had a new title, and Mega Man had a new title only 4 years prior to Smash, if he was irrelevant, then wouldn’t Shulk be, as his game came out in the same year and was a super difficult game to find. If they have newer titles on the previous console generation/ the in between of Smash games then they’re still relevant. It’s not like these characters are N64 or SNES relics, characters who haven’t appeared in like two decades.
2) Mega Man wasn’t neglected by Capcom. We really need to stop saying this, especially since we know what actually happened. It was clear that Mega Man was supposed to get some huge releases, including Legends 3, but that all changed when Keiji Inafune left. Clearly Inafune was making any production on Mega Man difficult as we’ve seen with Mighty Number 9, the dude loves to go out and try to do a million projects at once before he actually completes anything. Capcom parted ways most likely because Inafune wouldn’t just work on the game he promised, leaving the Mega Man series in a poor state as now the main director of Mega Man is gone. Capcom still referenced Mega Man in many titles and even supported a Mega Man and Street Fighter fan game, but losing a creator does significantly hurt a series, and since it seems to be more on Inafune, I can’t really blame Capcom for his death
3) Those characters you mentioned are the Retro Characters. Everyone in the Smash Community knows that Sakurai likes to fill one character slot with a random WTF NES character. Nobody ever sees the characters coming which is the fun of the pick. I doubt anyone really asked for any of the Retro characters as R.O.B. while important was just a broken toy that nobody thought would have a chance to come back with the only notable thing he’s done in recent years was try to kill AVGN, G&W obviously no one expected him, Pit was a weird choice as well and was clearly done so Sakurai could set up his next title as Pit’s design was updated. The only one people were expecting was Duck Hunt, but that was due to leaks and the fact that Sakurai had done it before with G&W and R.O.B., so a 3rd was inevitable.
I should state that Relevancy has no place in Ultimate due to the addition of K Rool, and maybe Ridley. If the character is a major player in terms of popularity than there is no reason they shouldn’t be included this time. I feel that the only reason relevancy is brought up, coming from someone who did use it, was because Smash 4 was sort of a dumpster fire and rarely threw a bone to old fans, relying more on modern characters than any previous Smash with all characters being somewhere in the Wii to 3DS era. It left the roster pretty uneven to be honest, so I’m glad the fans are filling in the missing members, but Smash 4 set a precedence for modern characters only with its many anime swordsman from the past 4 years, and lack of focus to any majorly established franchises except Mario. As Zelda, Kirby, Donkey Kong, Metroid, especially Star Fox, and others got pretty much overlooked. By the end of Smash 4, the moderns only became clear with Bayonetta joining due to her sequel, and the addition of Corrin, a character the West was yet to experience. I’m glad many of these characters are here, but it did leave a major burn on the community overall.
I honestly believe Brawl had the best newcomer roster in this regard. Combining old and new flawlessly to make one of the most interesting rosters, and putting a similar amount of focus on each franchise, unlike Wii U’s heavy Fire Emblem focus. In Brawl’s roster, we got classic characters like Wario, Diddy Kong, Wolf, Sonic, King Dedede, Metaknight, Charizard, Squirtle, Ivysaur, Pit, and R.O.B.. Then we got many new faces like Olimar, Lucas, Lucario, Toon Link, Snake (mainly known for the GC era entries), and Zero Suit Samus.
Sorry, I just wanted to give my perspective on the roster and how relevancy came in due to Wii U’s lack luster roster and huge emphasis on new characters.