I'd be leery of not letting the cap come down if league revenues come down, if for no other reason than I don't like how Stern's been running the League, I don't like how a lot of teams are run, and I have serious concerns about the NBA becoming more of an also-ran. Owners are also going to want some built-in restrictions on salaries if the top line DOES contract, the cap only being allowed to expand is far too favorable to the players.
Simple solution might be that if league revenues contract, the (hard) salary cap also contracts but not for a year or two; give teams a little time to free up space. The cap should also be based on a three-year moving average of league revenues, to smooth out some of the natural volatility.