Pretty sure the decline of MLB is due more to the fact young boys simply aren't playing it and interested in it, rather than changes to the game, which really haven't been very much overall. And the increase of popularity of football (pro and college). Biggest changes in MLB were 1. The DH, but only in the American League for most of its life; 2. Turf 3. Playoff expansion, which I think was detrimental, and 4.Now it's the pitch clock, which from my understanding nearly everyone prefers.
American culture changed too, speeding up as it is regarding time, and baseball games simply got longer and longer. Plus the strikes, PEDs, limiting starting pitchers on pitch counts, etc. Frankly, the linch pin for me was the one All Star Game that ended in a tie because of the fear of a starting pitcher going more than one inning and they ran out of players (thank you Joe Torre, you jerk).