I don't know how old you are but I can remember a time when you could only have one sub, that core group needed to be injury resistant/play though as best they could. Sure everyone gets a proper injury at somepoint and that's a bit different, but each player knew that if you have a prolonged spell on the sidelines then there's a possibility if your replacement comes in does a good job you are out the door. Without the mega bucks of today, that meant not putting money on the table.
Since the evolution of squads, pay, employment laws and the number of substitutions per game have increased it does appear that more players suffer from all kinds of niggles that your 70's/80's counterparts would have played through. Now yes this could be that they are more finely tuned and thus a bit more on the limit and they probably run faster and clock up more miles but it's also plain to see this generation are a bit more softer, weaker mentally than the older ones where generally you had to grow up a bit faster.
How much can be apportioned is up for debate but there is no doubt there is a link that these modern players that have good contracts, they get paid whatever and don't need to push themselves quite as hard as ones way back that would just be shown the door.