Vucevic is here until 2022-23 when he'll be 33 years old, at which point, he probably won't be an All-Star anymore and clearly declining.
Long term, yeah, it doesn't do much for us in terms of actually winning a championship. But it makes us better right now, and is better than standing pat with this crappy roster and uncertainty of both the draft and NBA free agency. If we were a gutter trash team with a chance at the #1 pick, I could see differently, but we have a very slim chance of the #1 pick without tanking the rest of the way.
Mid-1st round picks are not worth that much. It's like a 4th rounder in the NFL. You don't find a whole lot of stars mid-round, and if teams do, its generally sheer luck. Often any stars who have formed out of mid-round picks have taken years to get to that point (Kawhi and Greek come to mind).