I think this was a very good deal by the B's. I wish they could have gotten at least 1 player who could contribute right now, but ultimately it is a very good haul. While Kessel is an absolute sniper, he also had huge holes in his game, and reportedly had some real maturity issues. To be able to get those three picks for him is a very good haul.
It also gives them good flexibility this season, with the extra cap room. They could be very active at the deadline with all the picks they have in the next 2 years, and that extra cap space.
In addition, they also have some other nice young players, who may be able to step in this year. If someone like Hamill can step up and make the team, and Sturm and Kreiji prove to be healthy, then they really shouldn't miss Kessel that much.