Unfortunately it doesn't piss me off as much as trading away your first rounder then on a whim deciding afterwards its time to rebuild.
To be honest, the "rebuild" is not new, it was not decided around the Karlsson trade, it was started when they dealt Phaneuf.
So basically, what happened after a very successful 2016-17 season (98 pts, ECF game 7 double OT) :
1) November 5, 2017 :
traded Turris (
28 y/o, didn't want to extend him long term) + Bowers (recent 28th OA pick) + 1st in 2018 (top-10 protected but they were assuming it would be late-ish anyway) + 3rd in 2018 while dumping Hammond 1.5 M$ salary for an upgrade at center in Matt Duchene (26 y/o, a player they targeted for a while and planned to have long term). At that point, they still wanted to "improve" the team.
Everything was fine until they literally bombed the season. Then Melnyk lets go his frustration at the outdoor game and say regretful things, couple that with the shame of not selling out in the last playoffs, he orders Dorion to start cutting costs. That's where the "plan" changed (and you're right, it's crazy risky when you don't have your own 1st one of the next 2 years)
2) February 13, 2018 :
traded Phaneuf (almost
33 y/o) and
Nate Thompson (
33 y/o) for a player that would eventually end up on LTIR (so 80% insured) and a 7th rounder in 2019 (return for Nick Shore). That move save the Sens ~12 M$ in the next 3 years, assuming Gaborik stays on LTIR. But that's without calculating the cost of Thompson (1 year) and Phaneuf (3 years) replacements in the salary cap structure.
3) February 23, 2018 :
traded Brassard (
30 y/o) + 3rd in 2018 for Pittsburgh's 22nd OA in 2018 (then traded for 26th OA and 48th OA) + Gustavsson (a very good goalie prospect that was desperately needed) + Pittsburgh's 2019 3rd round pick + Columbus 2020 3rd round pick (return for Ian Cole)
4) June 19, 2018 :
traded Mike Hoffman (
28 y/o) and a 5th in 2020 for a replacement winger in Mikkel Boedker (28 y/o) + an average D-man prospect + a 6th in 2020. That move save the Sens 5.3 M$ in the next 2 years. That move is a bit different than others because Boedker and Hoffman have exactly the same age. However, we know the context was very unique with the relation between his fiancee and Karlsson's wife. But Dorion should have found a younger player to fit with that "youth movement". I'm thinking that Hoffman's value was really hurt and not many teams were ready to take a risk with a player who can bring problems in your locker room (see : Evander's Kane trade value)
5) September 13, 2018 :
traded Erik Karlsson (
28 y/o) and Perron (prospect who fell off the map and wanted out) for Tierney (24 y/), DeMelo (25 y/o), Norris (19 y/o), Balcers (21 y/o), a 1st, a 2nd and two conditional picks.
Others gone : Alex Burrows (
37 y/o), Johnny Oduya (
36 y/o), Chris Didomenico (
29 y/o), Gabriel Dumont (27 y/o), Fredrik Claesson (25 y/o)
Dumont and Claesson being the only "younger" guys gone but depth players at best. Everyone else gone is over 28 y/o.
If there wasn't for Hoffman and Karlsson, all of this made sense for getting younger and faster. And it's clear that you couldn't keep both Karlsson and Hoffman on the same team with the situation. One of them had to be gone absolutely.