This topic is often debated but I've found such debates are usually based on assumptions about what happened in the past, overgeneralizations and inaccuracies, and incomplete consideration of all of the variables.
For example, when you say pitchers pitched a lot more in the past, is that really true or is that an assumption based on hearsay? If it is true, are you talking about pro pitchers or little Johnny down the street? Kids did not have so many avenues to play coordinated baseball in the past. These days, kids have lots of leagues to play in including Little League, Pony, Babe Ruth, Ripken, etc. And then there's travel ball. Of course, the competetiveness has increased over the years - especially in travel ball. And kids are starting to play coordinated, competetive ball at earlier and earlier ages. On top of that many kids today play baseball year round instead of playing different sports throughout the year. So, it would be my guess that pitchers pitched less and stayed healthier at younger ages in the past than they do today. They weren't worn out and used up by the time they were adults "back in the day" like they are today.
I think that in order to really draw any meaningful conclusions, you have to take into account ALL of the variables involved. Otherwise, comparing today to yesterday is comparing apples to oranges.
I also agree with the comment that pitchers probably pitched through pain more often in the past than they do today because (1) we have better knowledge of pitcher injuries and their ramifications, and (2) at the pro and even college level there is more of an investment with financial ramifications involved.