Why is there no more longevity with today’s RB’s?

Most RBs are now smaller and quicker than those of the past. Also, they’re asked to catch more passes than any other time in history, which puts them at more risk of big hits from linebackers at full speed in open space.

This and I also think the lack of FB's have something to do with it as well. I remember the great FB's referred to as the RB's protector.....