The rights are not really with ESPN+ but more with MLB.TV. You can just subscribe over ESPN also.
The Nationals here are probably the basic model. $99 seasonal streaming package. A couple of the larger cable systems (Comcast and FIOS) have a dedicated Nationals channel that is active around games but does not show much other content other than repeats.
I think they had to offer this because they all had something like a $15 regional sports fee that got them both Monumental and MASN. They were not going to cut that fee so they had to create a cable channel.
Going forward I assume all the teams will be able to offer some games on cable or broadcast channels but you can also stream directly. Eventually it is probably 100% streaming but we are still years away from that.
I just wonder if a channel like NBC Sports Philly can survive if the sports rights revert back to the team.