Wanted to read the article, but I am not a WSJ suscriber.
In the old days, you could Google the article's title and get find it for free. Didn't work today. WSJ closed the loophole.
Damn Wall Street capitalists! Where's my free stuff?
Not having read the article.. I'll say:
Probably two things interplay to shorten shelf life:
1. the game is getting more physical
2. even the lowest paid player could live well for life on their NFL earnings (if they don't blow their money in foolish ways)
Or, put in other words: your 1960's AFL player was actually playing to make ends meet; they went as far as they could, injuries or not.
Today, a player facing life crippling injuries can say "I've got some money in the bank, I'm not going to risk my health when I don't need to."