Free agency is still ongoing and the draft is next month. Don't assume what the Broncos have today is what they'll have when the season starts.
Also, the Broncos were probably the most willing to give Peyton the kind of control he had with the Colts. Over the years he'd become the de facto OC and on-demand playcaller for Indy; everything about the offense was either his idea or tailored to his liking. The Broncos were going to have to start over anyway when or if they abandoned the Tebow experiment, so why not?
Not all who wander are lost