Ay so I know it's been a lot of hours since this convo happened but I was asleep the whole time and I feel like I really gotta say my piece on this
I think the problem here is that this is being looked at from a very extreme perspective. As in, "people only dislike the roster because of the lack of X-men and nothing else". Maybe that is the case for all the competitive peeps who are here for MAG-****IN'-NETO and FOOT DIVEFOOTFOOTDIVE, but speaking from the perspective of a casual fan who moreso came for the fanservice, I'd say there's more to it. 'Cause I feel like, in actuality, Infinite's Marvel side suffers from the same fundamental problem as the earlier Mahvels, but in a different direction.
All these unexplored corners of the Marvel universe you mentioned? I would have loved to see them! I would have loved to see Daredevil! I would have loved to see Kamala! I would have loved to see Green Goblin and Blade and Moon Knight and any of the F4 and Nico Minoru and Luke Cage! Problem is...Infinite didn't do any of that either. Because once again, I think that it in truth suffers from the same problem you have with the earlier Em Vee Cees, but we're failing to realize it 'cause it's not with the same group of characters. Just like, say, MvC2 would have you believe the X-Men are all there is to Marvel, Infinite would have you believe the beeg MCU headliners are all there is to Marvel. So it's not so much that its Marvel side sucks just because of no X-men or Doom, its Marvel side sucks because Venom is literally the only character that wasn't either A. ported from UMvC3 or B. added because of the MCU. In all honesty, even within the MCU sphere they still kinda did a sucky job. I mean, they still could have used characters like Scarlet Witch, Star-Lord, Loki, or Vision, but they went with Gamora? That's kind of the overarching problem with MvCI's roster, even in the Capcom side: aside from a few cool beans like Jedah, X and Black Panther, it has no spice.
All things considered, I even agree with you that we shouldn't have too many X-men. Thing is, i doubt many disagree. I don't think a lot of people are really asking for Marrow and Spiral to come back. Hell, I'd be fine with, for example, Logan, Storm, Pool, Magneto, one of Gambit or Psylocke, mayyyyyyybe Cyclops or a newcomer if you really wanna stretch things, and dassit.
Really, I get some of your complaints, but I feel like Infinite also has these problems - just in a different way - and it's not fair to nail it down to "well you just don't like it because it doesn't have X character". IMO, on a lot of fronts, and both in terms of Marvel and Capcom, it's just not good.