To me, losing Washington wasn't the end of the world. Bostick at least seems as capable as Isaiah in the limited minutes he saw last year. Losing Banks and Samuel is more significant, though I didn't like them as players much and they were role players (i.e. they don't make or break the team).
You hit the nail on the head though. We were always going to be reliant on Carr, Stevens and Watkins to make the leap next season. Ideally we would be able to sign another scorer, but that seems unlikely at this late stage. One more guy who can play guard and give some minutes would be ideal.