After just reading what the owners are proposing for the next CBA, scrap any free agency plans. 
to be fair, if the cap is that low, wouldn't all teams be in the same boat, theoretically leveling the playing field and giving a team like the Celtics (who do have cap flexibility right now) a leg up?
Can't see a hard cap as part of the new CBA. The league can't risk losing multiple, valuable assets belonging to small market teams in need of attendance. The soft cap works, and the luxury tax revenues should be divided evenly by teams who are under the cap. That way the larger market teams will be more conscious of their spending.
But you're point is spot on when you juxtapose Bos. to Miami. Celts have $30m+ off the books next season. Even with cap holds, free agent signings and JG's q/o, they're likely to stay well under the current $58m cap. More importantly they have Rondo's contract, which has trade value. We can only hope Green's contract would be as moveable.
Miami doesn't have that luxury. They're significantly over $58m for the next five years. Miller's contract can't be moved. Haslem is still owed $18m+, and we don't know if J. Anthony (4yrs/$14m) had a breakout year or it being just an aberration. Unless they use the MLE and split Jones/Chalmers salary they won't be able to keep either player.
So the Celts' do have an edge in cap flexibility, and without some palatable overtures by the owners the Heat will likely be playing 2 1/2 on 5 for years to come.