One of the reasons your seeing a lot of those good teams continue to get better and stay under the cap is because they are constantly rotating players out that are going to be up soon for huge contract raises for 1st round picks. Some of those teams already have 3+ 1st round picks for many seasons to come. Teams complain about them getting better and better and staying under the cap but at the same time feed into what is causing the problem. I have a few teams that are way under cap but I have stayed away lately from those huge FA contracts and as I have said many times, the cap can be manipulated when signing your own stud players to long term contracts if you dont plan on trading them. A fix to this not having the contract on your own teams players rely so heavilly on the total bonus but the overall of the contract.
Say you wanted to resign your stud HB and he has only been in the league say 4 years but he is good and improving. You go to offer him a contract and he wants say 5.5 mil for that first season and 4 years with 40% in bonus money. You can then offer him a 6 year contract and see what he wants for that contracts total bonus. All you need to do is up the bonus to 75% and then you can start lowering his total salary he wanted. In this way you may you could possibly save 2+ million a season on that contract. In this way you can still sign all your stud players to long term contracts.
You dont want to do this with players in decline or mediocre to bad players or even players you may want to trade as you will get hit with all that bonus money in dead cap space. In this way you can keep a very good starting team and be able to afford quality depth that can be rotated out every season or 2 even if they are in decline.
This is why I have been saying that more emphasis should still be on the total amount of that contract.