There should be no question that the C position will get significantly worse defensively with McCann gone, and most likely, the pitchers ERAs will increase. McCann leaving means more time behind the plate for Grandal who will be 1 year older, and it's most likely going to be Collins picking up the rest of the ABs. If it's Grandal + Collins + Yermin behind the plate, oh boy, we're going to be baaaad back there.
If there is one guy I would be 100% on board with throwing big money at this offseason it is Realmuto. Grandal's contract is moveable for sure and he doesn't have a NTC. Sure it could be seen as a "shrewd" move, but signing Grandal with McCann already here coming off of an All-Star season was already a shrewd move behind the plate. Realmuto is p4p one of the best players in all of baseball. He's the best C in baseball and would be a serious addition to our core. He's the best C in the game IMO since Yadier Molina, and Yadier IMO is the best one since Ivan Rodriguez. I would absolutely throw money at him. And I think there would be several teams willing to take Grandal off of our hands and still give us a good prospect because his deal is not bad at all and he is still a good player, just nowhere near as good as Realmuto.
Edit: I'll add one more thing: Realmuto as a C is a better longterm, big money investment than Machado or Harper would have been, and because he's a C, no matter how great he is, I don't think he can do better than 6 years guaranteed with any options after that being heavily incentivized.