Additionally, I only know of two ways a team can try to stay successful over longer periods.
One is to make an against-the-rules agreement with, say, a QB who also has his own business, where the QB receives a well-below-market salary from the team, but the team contracts the QB's company or makes other arrangements to compensate the QB outside the official salary structure. The money saved against the cap can then be applied to other players who can improve the team.
The other way is to keep drafting players who can contribute, including (especially?) at QB, and acquiring undervalued players via free agency and trades, in order to keep the roster from getting weak as the salaries of players contributing to the team's success increase and take up increasingly large portions of the cap. The problem with this approach is that some players get sensitive if a team is even looking at draft prospects at their positions, and fans who idolize those players get their panties in a collective bunch about the team "disrespecting" those players.