IMO just like with the THT trade, the simplest explanation is probably the right one - Ainge just loves Kelly Olynyk more than anyone else in the world. With old school GMs that's a recurring theme and one we should be ready for in the future. Ainge will make some head scratcher moves simply because he has his evaluation and doesn't go a step deeper and ask himself - are others seeing it this way and can I actually get more here? He was reportedly ready to trade 4 1sts for Justise Winslow. He also traded out of no. 1 pick when there was a consensus best player(and was right in that case), he also drafted Brown no. 4 when nobody wanted him this high. He just has his views and sticks with them even if unpopular.
I personally hate this deal. And the worst thing is - it signifies we probably won't be able to get anything of value for the other vets.