You're missing your own point.
It's clear from your post that fans are fans. You are missing that point. Most fans are not watching every game, let alone other teams. They simply cheer their team and trust the management - which is what the fans were doing.
It's not even shocking, that is exactly what you expect from fans. It wasn't the fans that were wrong, it was Dubas.
Now, fans realize Dubas is wrong and blame him... and you're turning around and blaming the fans for being happy their GM made a move at a time. It makes no sense.
Fans do not get paid millions of dollars or invest the kind of time and money or have the access a GM has. The fans can simply support the team or boycott the team - those are their options. What they believe has no bearing on the GM.
Your point is basically: "Fans thought Dubas was doing a good job when he wasn't and now they have no right to complain!"
Okay? Are they going to fire the fans or Dubas?
The fans do not invest as much time into the sports as you seem to think. Most fans watch a few games a year, of course they'll be happy their million dollar GM made a big move.
Basically, you think it's funny the average fan who watches 10 games a year was happy that Tavares was signed... and now they have no right to criticize Dubas for his failures because they thought Dubas made the right move at the time.
It's the weirdest argument I've ever seen.