So two obvious scenarios where everyone would have to say, "I wish we had traded Rondo when we could":
1. We offer Rondo everything we can and he still signs elsewhere.
2. We end up signing Rondo for more than he is worth and he never lives up to the contract.
Of course you would also be able to second guess any trade for Rondo if he goes on and carries some other team to a title or the players we get all flame out.
Point is I think it is just silly to make a proclamation such as "thou shalt not trade Rondo" or on the other hand, "we must move Rondo now".
I actually believe that on balance, trading Rondo (assuming we get a decent package) would be overall less risk and more likely benefit rebuilding than letting this play out to free agency. I think if we don't trade Rondo, the best we can do is to overpay and of course the worst is we end up with nothing.
Now of course if you think Rondo is a max player and can be the best player on a title team, than your solution is to just offer him the max and expect he won't leave. Fill in around Rondo with draft picks and the rebuilding is done.