Why would MLB do that?
They probably won't, but the RSN via cable/satellite is a dead business model walking. I can see them as a standalone streaming service with a monthly charge for each network, but the days of partnering up with other providers is on its way out. In the case of DirecTV, it's also on its way out.
For example, in metro Phoenix, Fox Sports Net Arizona is only available to about half the market, via DirecTV and the cable companies. Dish and YouTube TV have already dropped them. Fortunately, they aren't airing anything important right now, but the NBA is starting up again next month. Once baseball season starts again (with hopefully a full 2021 season), they're going to have to figure something out.