The Broncos have cap space.  They are in the middle of the pack in terms of cap space, with a lot fewer needs than most teams in better cap shape.  The Broncos could sign two top free agents with ease.  They aren't going to be big players in free agency because their team is mostly set.
5 mill more from Peyton would be useful, sure.  But if they need 5 mill more, they can do it without asking anyone for a pay cut.
They have a franchise tag on Demaryius, is that a luxury that a team desperate for cap space can afford????  Sign him long term, and you can drop his cap number for 12 mill to about 5 mill.  Von Miller has a 10 mill salary.  Convert salary to a signing bonus, and gain another 5 mill.  This team could get to 40 mill in cap room with ease.  Could it potentially be kicking the can down the road?  Sure.  Who cares.  This team is the definition of '2015 or Bust'.
The Broncos could keep most of their free agents if they really wanted to.  They are choosing to let Julius walk, they don't think he's worth what he wants.
Where is the crying need for cap space?
More interesting:  Where's the national stories on Drew Brees, and his bloated cap number?  The Saints are about to start tossing bodies off the ship, and he has a 26 mill cap number.
What about Stafford?  He has made more in salary than any young QB ever.
		
		
	 
Elway has been consistent with his message from day 1 that his philosophy isn't about winning now, it's about winning from now on.  He does not, and has not, structured contracts with big signing bonuses, kicking the can down the road.  That's not how the Broncos function, and it's why they are always in good shape cap-wise.  3 years in a row, they have landed key free agents and it's because they aren't strapped with ballooning salaries or dead money from older contracts.  I'm talking about guys like Talib, Ward, Ware, Vasquez, Welker, Knighton, Sanders, DRC, Phillips, McGahee, Bunkley, etc.
Every year since Elway took over, they came out strong in free agency.  Fans of other teams ask how they are able to do this, and it's because Elway has let big salaries walk when play diminishes (Champ Bailey, Dreessen, Stokley), has let replacement players walk rather than fork over big money (Decker, Ayers, Moreno, Woodyard, Bunkley, DRC), and has asked vets to take a haircut (Manning, Dumervil).
They aren't going to restructure DT to a $5m deal with a huge signing bonus, that's not how Elway does business.  If you want to see an example of how his contract will look, take a peek at 
Ryan Clady - the last guy the Broncos franchise-tagged.  He signed a long-term deal - 5 years, $52m, with a $3m signing bonus.  his first two years, his dead cap # was slightly higher than his cap hit, but for the next three years the Broncos could cut him and save about $10M in salary with minimal dead money.
bottom line - Elway will not allow the Broncos to go to salary cap hell in a couple of years for a title shot now.