The returns weren't bad, they just haven't worked out.
Nahhhhhh
Anyone who paid attention knew that Weber was going to have more and more trouble with his mobility and inability to move the puck effectively in his own end. We know he has a bomb of a shot and is productive offensively, we know he's tough along the boards, but we also all knew his limitations, the fact that he's 4 years older than PK, and that he has a ridiculously long and dumb contract that guarantees a big cap hit until he's almost 40 unless they get a compliance buyout.
It was a "dump PK" trade, not a "let's acquire this awesome player" trade. It was a better return than B prospects and picks perhaps, but there were plenty of warning signs that it wasn't going to work out favorably player for player for Montreal.
The dumb thing about trading Sergachev for whatshisname is that they really wanted a talented center but Drouin really hasn't been much of a center to this point. I don't totally blame them for that trade and Drouin could still pan out to be a real impact player for them, but their motivations were a little off IMO. Plus it continued to decimate a defense that lacked any mobility or creativity.