IMO you are missing the fact that there is a high probability 31 year old almost always hurt throughout his career Mostert gets injured again and is not available to us. So our offense can covet speed and we will have Achane but if no Cook and Mostert goes down your RB is now Jeff Wilson for the year with Achane getting some change up RB snaps plus whatever receiving packages McD is actually planning for him. We talk about OL depth all the time, wouldn't we like some RB depth? A proven vet stud like Cook to do the heavy lifting with your old/fragile but fast and explosive vet moving to a change of pace role much better suited to his aging body? Wilson instead of an ankle sprain away from being your starter moved to the #3 role? Achane totally free to be used in whatever package McD likes and not have to depend on a smaller sized rookie to carry say half or more of your traditional RB duties if Cook is not around and Mostert were to go down?
Then add in the fact if McD is as smart as we all think, he is planning on running the damn ball more now to protect Tua's health and really the #1 factor in whether we will compete for a SB this year or not. They have to be planning for more balance and even protective game planning at this point and again, not to beat this dead horse but we need the depth. I really like Mostert and how he plays but you can not depend on him, as a fantasy owner that year after year watched him go to the shelf you just can't. We got the rookie speed playmaker, we have an unspectacular yet reliable steady eddie vet in Wilson, and we do have Mostert, for now, but I think you have to plan as if you don't have Mostert and get another guy so you hopefully always have at least one of them ready to go with the other supporting pieces.
Tua's health is the #1 priority this year and the best way to do that is with the run game. For a team on the verge of really competing and given the makeup of our current RB room I think Cook makes a lot of sense. I hope they can work out the figures. I can tell you our AFC East opponents will not be pleased at all to see Dalvin Cook lining up in our backfield behind Tua with Hill on one side and Waddle on the other and any number of skill guys in the slot (Achane, Berrios, Anderson, etc). Good luck defenses playing the chess match against those pieces all game long.