Let's look at the best parts of the trade.
The Sabres do not retain any of Jack's contract.
Sean and Erik's salary will keep the Sabres above the cap floor.
He is traded to a western team.
This the equivalent of over four first round picks.
Monahan and Dube are equal to about two first.
Zary is at least a first. He will be challenging Casey and Dylan for ice time next year.
Plus a 2023 first and 2022 second.
Whether Pysyk is better, Gudbranson is not a top four RHD but at the TDL when GM's are looking for veteran depth d-men, he would return a pick.
Let me add, he may be just the Defenseman who could help Rasmus reach his potential.
Other than the shortened seasons and 2016-'17 he has played in at least 50 games every year.
Finally, Pysyk could also play RW as Buffalo only has Thompson, Hinostroza and Okposo RH RW.
I see absolutely no downside to this trade.
Picks (first and second), prospects (Zary and Dube), veteran players (Monahan and Gudbranson).
Punches three of the four areas the Sabres need future help, Center, RW and picks.

Now I must ask Ledge_And_Dairy what do you know that caused you to add a second and Gudbranson to your standard package?

EDIT:
Other GM's have a wanted him.
2019-Feb-25 Traded from Vancouver Canucks to Pittsburgh Penguins for Tanner Pearson,
2019-Oct-25 Traded from Pittsburgh Penguins to Anaheim Ducks for Andreas Martinsen and round 7 pick in the 2021 draft
2020-Oct-08 Traded from Anaheim Ducks to Ottawa Senators for round 5 pick in the 2021 draft
2021-Apr-12 Traded from Ottawa Senators to Nashville Predators for Brandon Fortunato and round 7 pick in the 2023 draft
I think he would return at least a fifth at the TDL, would love to get Pearson, LOL.

Im curious how that's going to work since they need at least 46 to run both leagues