Very few are sure fire prospects, especially goalies. I have been encouraged by Lehner's season, however he is still relatively unproven and has a history of concussions. Ullmark has also shown some flashes but is also unproven. Having another guy with potential in the system would be very beneficial.
I remember when the Sabres thought they were all set in goal having Mika Nornen, Biron and Miller in the system. Nornen and Biron were both picked in the first round, while Miller was a 5th rounder. I remember the organization being much higher on the former two then the later. My point is, yes this is not like losing Vesey, but I'm very hopeful he stays in the Sabres system. You can never have too many promising goaltending prospects.