How do these people get coaching certificates?
Carping from parents about playing time probably dates back to the advent of team sports. I just think it's easier for pushy parents to get rid of a coach now at the high school level because you no longer have to be a teacher in the district to be the coach. That makes for a larger pool of candidates, some of whom have no protections at all if a change is desired. Anyway, that's my take on it.
As for kids today, I don't think they are any more or less tough than they were when I was growing up in the 1950s and 1960s. The biggest difference I have seen is that parents and coaches are both "in their crap" a lot more when it comes to athletics. Kids today seem to always be looking over their shoulder instead of developing their instincts. Again, just my opinion.