1) As pointed out earlier, good FAs rarely come to Boston.
2) Yes, some players do change teams. And there is a good chance we might get a good player in a trade- just like with did with KG.
But:
i.Draft is by far the cheapest way.
ii. There are many all-star level players who stayed with their teams for ever. Not just for the money but also out of loyalty. Nowitzki, MJ, Bird, Magic Johnson, Stockton, most of the SAS roster, and the list can go on...
The point is not that draft is the one and only way to build a contender. But that it is one of the best weapons Boston has in the present situation.
If we were talking about LAL or Miami it would be a very different story...