I think it is a fairly reasonable trade for both teams. Lot to give up, but Towns seems like the type of player that Ainge might sell the farm for. I'd probably try to get Bates-Diop back in the trade to add some depth.
Post-Trade
PG's - Walker, Wanamaker, Edwards
Wings - Brown, Hayward, Langford, Bates-Diop, Semi, Green
Big - Towns, Kanter, Williams, Williams, Poirier
I think that trade puts Boston up in the Milwaukee tier in the conference as it balances the roster out more.