"He has more room for development than any player in the NBA" pretty much means he's far from being the player you hope he will. There's no guarantee he gets there.
Again, was Rondo the player we all hoped we could become when he was 19? There are no guarantees in life, with either player or anyone else.
I use these type of trade ideas to gauge whether peers are prepared to sacrifice (or not) to rebuild with compelling talent. There's little chance a deal like this would actually happen, especially from the Bucks side. I did the same with the "Rondo for the 1st Pick in 2014 Draft" idea, which nearly everyone saw as a layup for the Celtics. I'd argue this deal is a better one for a number of reasons.
You see it as too risky for the Cs. Fair argument.