IMO, the credit goes to John Elway. Yes, it was an "all in " move, but it was structured such that the Broncos could back out with minimal impact at any time. Manning's release at any point his whole contract would only cost $2.5m in dead money. His contract overall was not far out of line from other good QBs.
It was "all in" as far as building an offense around him, but there are few NFL players worth scrapping what you do for one guy...PFM is one of those guys and it was worth every little bit (until it wasn't, of course).
Manning's retirement does not leave the Broncos on a big hole, IMO. I don't know if Brock will re-sign or if they have to get someone else, it really doesn't matter. This team just won the Superbowl with some of the worst QB play in the league, so I feel good about it either way.