Derrick White is a really strong player -- yes he's not the best shooter, but he can get his own shot, distribute, and defend well. He can do it with more volume than Richardson. And he has some positional flexibility. The contract is rich but it provides stability with its length.
It's a real shame to lose Richardson, I enjoyed him for the same reasons but he's more deferential. The first round pick is a real overpay, in particular if Boston slides back down a few slots in the standings. It's a real bet on this year's performance.
You have to hold out judgment until the full trades cycle through, and you have to assume there's more to come.