The thing about Warren that can, and has, upended more usage: ball security.
He has more juice than given credit for (certainly more than Naj ever had), is a pretty good pass blocker, and has turned himself into a better than advertised receiver, BUT...he puts it on the ground at times (more career Fumbles than TDs). Tomlin HATES that, possibly more than any current HC (and ALL coaches hate it). Conversely, Kaleb Johnson has had, like, 3 fumbles in his life. That probably gives Mikey T "pants feelings."
I like Warren. Health permitting, he is almost assuredly going to get more run(at least early), has good pop on his first step, and he is the guy most likely to get the garbage time catches/etc in that backfield. My guess is he'll cede nearly all SY/GL work, and potenrially a lot of "high pressure" carries, due to the security issues tho.