But doesn't The Show still have the rights? Or can EA opt in too and thus, we will see a resurgence of MVP baseball? 
The Show was able to continue making games because 2K only bought the 3rd party rights, which meant they were the only non-system owned developer that could make MLB games. Since Sony is obviously owned by Sony, they were allowed to continue to make The Show.
With 2K's license coming up after this year, any company is now free to make an MLB game again, as long as they pay for the MLB license.
This differs from Madden/2K/NFL because Madden bought all of the rights to NFL games, so it didn't matter if you were first party or third party, only EA had the right to make NFL games.