There is nothing more demoralizing than a team that can RTFB and you can’t stop or slow them down. The issue is durability and in this day and age it’s hard for a RB to survive the pounding. But I agree a back that is big and physical and can catch that’s a unicorn. Think someone like Johnathan Taylor.
To me, the most valuable player on defense is a LB that can hit and tackle and cover or a DB that can do the same. You don’t have to mix up your schemes or sub them out.
Offenses have done the same thing with the versatile TEs like Gronk and Kelce and now Kittle. Feature a guy that can block and run and catch and it’s hard to match up with them.
Quarterbacks are the most important position on the field until they hit that big contract and it’s hard to build enough talent around them to survive.