I think, before the last couple of years, the RB had lost a lot of value. Now you are getting players like Bell, Gurly, etc. who can redefine the position and it is becoming more important. How much more valuable is it to be able to have a 3 down back? Seems pretty valuable to me not to have to shift your RB player and still have the playbook wide open.
When compared to the 4 "best" RB going into the 2017 season, Bell's stats for the first 4 games were not significantly different than the other 3. You might say, infinitely, better than David Johnson....
And thats the issue to me wanting to compare him to the other backs who dont do what he does..