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

A few possibilities:

1. Selection bias -- as others have mentioned, you're using HOF backs as your basis of comparison. We still have/had some of those in our era -- Adrian Peterson, Frank Gore, etc. Are they the exception? Of course, but they were probably the exception in prior eras too. Surely the 1970s had guys along the same lines as Knowshon Moreno, or Doug Martin, or BenJarvus Green-Ellis -- guys that made a splash in the league but quickly fizzled. But we don't remember them.

2. Different usage -- RBs today are asked to do a whole lot, including playing WR. Elite athleticism is more important.

3. Teams run less -- this doesn't mean RBs break down earlier, but it does lead to many superstars like Leveon Bell getting shuffled around the league, because the team that drafted them doesn't want to pay them big money -- and oftentimes this leads to them joining a bad organization, and their career dwindles.

4. Maybe veteran RBs were never a good investment? Maybe modern NFL teams have just learned from experience, and are smarter about moving on before it's too late. I'm not sure I believe this one, but it's possible that teams should have moved on from the Emmitt Smiths and Eddie Georges of the world sooner than they did, but kept paying them big money for nostalgia reasons. For example, Smith never topped 4.0 yards per carry during his last four seasons. George averaged around 3.2 for his last four seasons.