Until we hear the word "exclusive" I think we have to assume MLB.tv will not be exclusive to ESPN. The dollars needed for that would be far more (billions) than have been suggested in the press since ESPN really wanted to cut the $550 million they are currently paying.
This new deal is probably for a fraction of the past deal. Midweek games are less valuable than the Sunday games. No playoffs or home run derby. But maybe $100 million or so to allow ESPN to offer this package on this service where the money will likely be made selling personalized ads to viewers - something that MLB is probably not cutting edge on.
Now ESPN is making the app available to most of their cable customers, so the barrier of getting it may be smaller, but I just don't think they see this like Sunday Ticket as something that will attract loads of new customers. MLB might lose half their audience if the package suddenly became exclusive to ESPN and that is definitely not their goal here. They need more viewers not less. The whole Paramount UFC deal was about gettiing more viewers too for that sport.