Two quick points:
1) Coming to an aging team isn't exactly appealing for Howard. Maybe he wins one title, but after that, what's his future like with Rondo and him as the core? It isn't bad; however, if the cap shrinks under the new CBA, they may not be able to add another star to the mix.
2) I agree though that Howard might be the most realistic chance for us to win another title anytime in the next 5-10 years. And I don't think that getting Howard is all that realistic. But look at how you win in the NBA: you either get lucky in the lottery (look at the Bulls with Rose, Thunder with Durant, or Magic with Howard) or get lucky with free agents (yes, you have to clear the room, but you have to get fairly lucky that the player chooses you).
With the Heat, Bulls, and Thunder all very young, even if Ainge adds some good pieces to Rondo and Green, it likely won't be enough. They're either going to have to get lucky and land someone like Howard or they're going to have to get lucky and win a draft lottery at some point in a good draft. And the chances of the latter are even smaller than signing Howard.