if any umpire thinks the ball is just clipping the stumps then they shouldnt be giving it out, they could only be guessing at that point.
Why not, it ball clips stumps during out bowled, it is out, same for lbw, as simple as that. If it misses stumps, DRS is there to save the batsman.
I am more worried about umpire margin of error (which is inconsistent) than DRS margin of error (which is consistent and fair to both teams at least) and umpires call having to stay despite DRS showing otherwise.