The problem, for me, isn't really the 7. They can work around it. I think there are issues with folks from Westchester County and north, people in the Bronx, and many people in Manhattan. I don't know Brooklyn well, so maybe someone else can talk about that.
The best part about Yankee Stadium is how accessible it is from literally any direction, is accessible from both sides of Manhattan and has 4 separate trains that run there. That means that fans have 4 different modes of public transportation to get to the game, not including buses. Cutting that by 75% is going to stink, no matter how well the 7 runs. Everyone will be rushing to the concourse for the same train. Unless we get some bus shuttles from different parts of the city, I can see it being too hectic.
im in manhattan and i can get there just fine just one trasnfer, assuming trains work right. Though i do see the concern for anyone using the Metro north since they would have to switch the the MTA 7 train to get there ( though anyone currently coming from LIRR would probably do the same) . There would be massive overcrowding. though and that can be a problem.