It's possible there are multiple influences at play here. I'm certainly not disagreeing with you, you make some very good points about accessibility of content. And I'm also of the opinion that baseball is deeply uninteresting to watch. I can understand how someone could be into it (much as with any other hobby I don't partake in), I just personally find it only marginally less dull than a seminar on comparative accounting practices (read: a great deal less dull than cricket).

I think a big part of it is the diversity of entertainment we have available now. If your interests don't align with what baseball offers, it's no longer a problem to find something else to occupy your time with. You're not trapped into a paradigm with five or six sports to choose from, each with a limited season, and many of these new ones you can also engage with directly (gaming, drone racing, CTFs, competitive nerf battles, etc.) which gives you an appreciation for the game that is missing from some professional sports. Take Basketball and Football: both are still quite popular with the younger generations, and both are physically very integrated into american culture. Streetball is about the most accessible sport out there, and every school in the country has a football field (and you can play touch or flag football games in any park)

I suspect it's the same reason non-american Football (soccer) has maintained such popularity: there is almost no barrier to engagement, even at a non-professional level (you just need a ball, a couple piles of sweatshirts and some friends) and more developed infrastructure for it is incredibly easy to find the world over. Whereas baseball, tennis, jai alai, golf etc. are all unsafe to play in a public setting where there's a risk of an unaware bystander getting beaned by a small hard ball going 200mph, and require safety equipment that raises the facility cost (and thus barrier to entry) by quite a bit (ex: nets). They still have traction, but if you're a kid in a shitty suburb or poor town, you're far more likely to be able to play soccer/football/basketball than you are baseball, and will be able to relate more intimately with those games when watching them played.

(And that's not to mention esports)

When we've got so many choices and so little time to ourselves, why spend it on something we have to compromise our way into enjoying or that is a particular labor for us to be able to consume, thanks to the fragmentation of streaming rights?