I think the "decline" of baseball is just a parallel to the general dissolving of American monoculture. Sure, it's become more niche over time, but so has every form of entertainment aside from maybe superhero movies. What's the last book that everyone you know read? What's the last TV show that everyone you know followed? What's the last band that everyone you know listened to? When's the last time everyone you know was getting their news from the same sources? What's the last board game that everyone you knew had in their closet?
Sure, football and basketball are more popular than baseball today, but today I know more people who don't follow sports at all than I know who follow football. Baseball isn't king anymore, but being "king," for a sport, means much less than it ever has. Americans love having choices and hate rallying around one big thing. I think it would take a huge cultural shift for Americans to HAVE a national pastime again. I predict that the future will see further fragmenting, with the big four sports all losing some ground, and more people taking interest in soccer and (shudder) e-sports.
Baseball isn't going the way of boxing, it's going the way of Superman. Sure, you can point to Batman or Spider-Man edging him out as the most popular superhero, but what's really turned him into a niche character is dozens and dozens of other niche characters.