You have to trade Crowder, Smart, and 3 non prime 1sts for George every single time. Look at this way, Smart or Bradley is likely gone next summer because it isn't financially feasible to keep both. Crowder may be starting now but he is pretty clearly a bench level player. Assuming the 3 firsts were 3 of these 4, Bos 18, Bos 19, LA, Mem then there is no reason to keep those as they likely won't make the team long term anyway.
Even as a rental you have to make the trade but there is no guarantee it would have been a rental.