How is the RSN losing anything? In fact they would still be on cable/satellite, AND have more viewers by watching them online. And I'm not so sure they get more from cable than advertising, in fact I doubt it. Not only is there actual breaks for advertising but maybe more importantly there are in-game advertisements, such as "The first pitch is brought to you by..." Also you make it sound like it would be hundreds of thousands of people, it only would affect the ones in the actual DMA the ones they really want to reach. It could require an authenticated sign in just as most online channels do now, or give a discount if you have cable. It's antiquated to think you can't be online and on satellite.
I'll even go a step further, if Baseball wants to capture more of the younger viewers having the RSN broadcast on MLB is one way to do it. Despite your characterization about the worth of MLB now, it is actually very well received, is subscribed to by alot of households. Last I saw it has 1.5 million or more streams a day during the season. It is such a hit that HBO did not use their own in house service people, and used the the ones who did the MLB TV site to make the new HBO site.