RBs get hurt in the NFL on every team not just ours, and teams mitigate that with depth. We had enough depth this year but the timing of everybody being hurt at the same time was unfortunate. Most of the good teams are looking at huge salary cap issues this off-season so we're actually in relatively good shape there. Carson's injury history will be priced into his contract and so there's no reason for us not to be involved in the bidding unless it gets crazy.
However, it's less about quantity running the ball and more about quality. The main concern with the 2020 offense is that we were unable to capitalize on teams playing soft defense. Increasing the threat of our running game is one way to make teams bring more defenders into the box to stop it, and then that opens up everything in the passing game for Russ. It would be fine if we never ran the ball as long as other teams loaded up the box, but our issue was too many times needing to hit intermediate and long passes against teams that were just sitting back and waiting for them.
There are other ways to improve the running game besides simply adding RBs. There's room for improvement with our run blocking, not just from the OL but also from the TEs, WRs and maybe a FB. Reducing penalties is maybe the biggest factor, as nothing kills a running game like a holding call. We can also improve the running scheme itself, and/or adjust the passing scheme to capitalize on safe short passes that the opposing defense is giving us.