I'm almost certain that Marvel will use Superman in some form once he is public domain. Whether it's in a one-off like the Superman vs. The Amazing Spider-Man crossover or as an active superhero in the MU, I don't know. But I highly doubt they won't use him at all, especially when independent content creators start using him, and they definitely will.
If they do bring Superman (and eventually Batman) into the MU, a more interesting question is if they'll eventually bring in Metropolis and Gotham into the MU once those cities become public domain. Marvel for the longest time has marketed itself as "the world outside your window" and their characters have usually operated in real cities. Adding in two metropolitan cities the size of and identical to New York would change a lot of things in-universe. And on one hand Marvel will want to maintain their "world outside your window" marketing shtick, on the other hand Metropolis and Gotham are characters on their own and are too interesting & profitable for Marvel to just discard, especially Gotham which is its own profit-making brand name at this point. The same can't be said for DC's other fictional cities. Other cities at the very least aren't very iconic and profitable even if certain writers have done interesting things with them. So how those two cities joining the public domain will impact 616, that will be very interesting to see.