Baseball not on the decline as far as organized leagues go but as far as kids playing baseball on their own at a field without adult supervision has been on a huge decline for years. You don't see fields where kids play unless it's being run by an adult.
Most of my ballplaying as a kid was unsupervised and teams were kids only. That is almost non existent today. For a gang of 20 kids to grab balls, bats and gloves and meet at a baseball field is foreign to today's kids. Of course part of it is lack of decent fields to play on but then again as kids, we found places to play. Another thing today's kids have no concept of.