Baseball looks like it's doing pretty well over the last 60 years or so. Average attendance per game 3 time higher than it was in 1950.
Source: http://www.beyondtheboxscore.com/2014/2/10/5390172/major-league-attendance-trends-1950-2013
Baseball is more popular than ever in terms of fan support, it’s just that now there is so much more competition in sports to follow. Back then the three most popular sports were baseball and boxing, and college football. The NFL back then, was probably about where the NHL is today in terms of popularity.
A big savior of baseball are regional TV broadcast deals, though the Dodgers got too greedy with the LA SportsNet deal. Note none of the other regional broadcast deals are anything as disastrous as what the Dodgers tried to do.
Football is a Goliath. But remember what happened to Goliath at the end. ;)