The problem with trading for even an above average starting QB is that the Brancos will not be the only franchise looking for one.Almost half of the league doesn't have that caliber of a player. So even with a bit of competition, the trade value will end up getting inflated to unreasonable levels. eg. Jay Cutler got traded from the Broncos to the Bears for Orton, 2 first round picks in 2009 and 2010, and a 2009 3rd round pick. Totally not worth it for Chicago.
Elway should have learnt from the mistakes of past teams that lost great quarterbacks to age or injury and drafted one in advance, especially one as good as Bridgewater. Because Manning was always going to decline sharply once his body refused to keep up, he has had multiple surgeries down the years and is almost 40. The whole Steelers dynasty collapsed in no small part due to Terry Bradshaw's elbow injury in 1983. Ironically they had a chance to take Pittsburgh native Dan Marino in the previous draft but passed on him. Green Bay played it perfectly IMO, got Rodgers and sat him behind Favre for 3 seasons and now they're reaping the benefits of astute future planning.
Good news for Denver is that there are a couple of solid options in 2016 FA just like in 2012 when they signed Peyton - provided the QBs aren't franchise tagged.
http://www.spotrac.com/nfl/free-agents/2016/quarterback/
Rivers is reportedly stirring in San Diego because he 'doesn't want to move to LA'. He would be an immense signing all things considered. From a division rival, Top 10 type of quarterback, experienced, can play for about 4-5 seasons still. Kubiak is renowned for his ZBS running and Rivers never had a good running game with the Chargers after LaDainian Tomlinson retired. He could do a lot of damage with the Broncos. Also Eli, who still doesn't have a new deal with the Giants. If he isn't tagged then getting him would make a lot of sense. Maybe he could go on another fluke run and win them a Superbowl.