Let's say the Broncos add a couple mid-level players in Free Agency this year (Terrance Knighton and Antoine Cason) adding up to about $6.5M.
Even if Dumervil goes down to $8M per year (highly unlikely), the Broncos would roll into 2014 spending at the Salary Cap already (~$123M)...with Beadles, Walton, Decker, and Chris Harris going in to Free Agency...and possibly Von Miller needing a raise.
To be able to keep those players in 2014, Peyton Manning is going to have to take a paycut, that's all there is to it. Either he wants to win a Superbowl or he doesn't. One thing is clear, the Broncos don't need to be gutting their roster of young talent the next couple years just to keep Manning on at $20M. No other QB's who want to win do that. Once Manning is gone, the Broncos will have to start from scratch in the draft all over again.
Let me just say that paying Prater close to $4M per year was a very bad contract for the Broncos. Big mistake. They need to draft a kicker and cut Prater as quick as they can ($2.7M cap hit). He's one of the major reasons the Broncos lost in the playoffs anyway.