Maybe it's as simple as McVay realizing that the run game is stalled without a lead blocker, i.e. FB or HB. Simple film study would show how many runs would have been productive had there been a lead blocker who can blow a LB out of a gap. Short yardage is where it shows most. Stafford sneaking is dumb. Two players the Rams cannot afford to lose are Stafford and Kupp. Very poor resource management to continually expose them to injury for a yard. The 3 inside OL open the gap but without a lead blocker, it's idiotic to expect the RB to break the tackle of the ILB who moves to fill the gap on every run.
In McVay's offense, the passing game is used to set up the run attack. That's not what he wants but that is the end result of poor run play design. As long as the MIKE stays put the Rams can't run inside without a lead blocker. That's why I've said the passing attack must be used to shift the focus of the MIKE to open the run instead of the reverse.
McVay wants consistency in the run attack but hasn't done enough to enable it. FBs are cheap so they aren't exactly cap breakers. A FB can be your #4 RB, i.e. a part-time player. A 7th round pick or UDFA having the desired traits to be coached up in blocking technique. A small price to pay to jump start the run attack.