There are many reasons why 2v2 is not good for Marvel 4 or a step back. I can go on in detail but 90% of people here will not care because they care more about their favorite character making in than game mechanics. This is not a diss to them, everyone has their preferences and priorities... I want as many people to revel in the hype of Marvel as possible even if you don't play the game.
2v2 with assist is nothing new to the VS series. It was done in Marvel Super Heroes vs Street Fighter where your partner came out as the assist. 2v2 with assist was done again in Marvel vs Capcom 1 only this time you chose a 3rd assist character as your only assist.
These are not the most popular VS games and while the assist system was perfectly fine back in the day the series continued to evolve and culminated in the game that is Marvel 2.
3 on 3, two assists that your partners have to do. No longer do you play 2 characters one by one... now you are playing a team. Synergies matter far more, team order matters far more, assist selection and combinations matter more. It was a paradigm shift for fighting games and it gave birth to actual fighting game theory crafting that is more advance than just analyzing match ups and character strengths. There is now more risk to calling assists because you can lose health/character off of bad assist calls. It also gave birth to character positions and roles.... stuff like point, battery, support, anchor, secondary etc.
It was an evolution of the VS series in every sense of the word but Marvel 2 did not perfect that formula. Marvel 3 took things further by adopting the formula and despite many improvements it still has not maximized the potential of this system.
When people talk about Marvel these days they talk about player freedom, creativity and hype. Where does this comes from? It comes from team construction and execution. Someone constructs a new team that people have not seen before and cements it as HIS/HER team and no one can play the team like that. That is freedom, that is creativity and that is hype.
You look at the top 8 or so Marvel players in Marvel 3...ALMOST NO ONE PLAYS THE SAME TEAM! Justin plays Wolverine/Storm/Akuma, ChrisG Morrigan/Doom/Vergil, FChamp Magneto/Doom/Phoenix, Cloud805 Zero/Dante/Vergil, KBR Hulk/Haggar/Sentinel, Clockwork Vergil/Doom/Strider, Dual Kevin Deadpool/Dante/Hawkeye, Apologyman Firebrand/Doom/Skrull.
These are different teams with different strengths and weaknesses along with different strategies and win conditions. The match up permutations are endless and more importantly variable within a match. Oh noes, Magneto beats Haggar 9-1 KBR will never beat FChamp... psyche he whooped that candy ass in grand finals of ECT and got the EVO title before that. Why? Because that is only 1/3rd of the matchup, even less because assists are involved. You can't look at Marvel match ups as other fighting game match ups. You can't look at Marvel mechanics as other fighting game mechanics. What works in a 1v1 anime fighter may not work in a 3v3 fighter like this... Hell same for 2v2 fighters.
It's its own beast. It's complex, it's hard, execution heavy, punishing as hell, the combinations and permutations you have to know are vast, it's not a game that you can pick up and expect to do well at. As Yipes puts it, you will spend 2 years getting your ass whooped at the game and then maybe you stop sucking ass. This is what makes the game hype and compelling for fans at high level. Forget what you think you know about the game when you watch Foot Dives, Morrigan fireball spam, TODs, Missiles, X factor comeback... there is a whole lot going on in the background and getting to that stage that is difficult for most people to grasp. It's a hard game to play but so are some of the best games like Smash Melee.
It's not a fluke that Marvel 2 and Marvel 3 survived this long (unpatched and unsupported) where as other 2v2 fighters came and went.