It doesn't have to be a supporting role, just not an action-driven role. Naturally the story structure would be reworked to focus just as heavily on the boss as on the field team. We've already seen that happen to an extent with Alec Baldwin, whose role in the previous couple of films was somewhat larger than the team's superiors' roles have been in the earlier installments. It's just a matter of taking that a step further.
I mean, of course Cruise is going to want to keep doing insane stunts as long as he can, but the time will come when even he'll have to admit it's not feasible anymore. This is a way to keep him involved in the series while adjusting to his age.