If you move Karlsson now you're essentially starting a complete rebuild. You can't really do that though if you've already thrown away your 2019 1st round pick. Dorion has really managed to move this franchise into a dumb spot with his strange management.
Personally I'm still convinced that Karlsson will be good again - maybe not "last playoffs / best player in the league" good, but certainly much better than right now. In actual fact I believe that next season the Sens can be a fringe playoff team again with a better Karlsson and the current core if you subtract a few useless chumps (scratch Burrows etc) and add Chabot, White, a more established Harpur plus possibly Brown or Chlapik or whoever the Sens pick with their 1st round pick 2018. They won't be a contender or a guaranteed playoff team, but we could see a re-tooled, younger and hungrier team lead by Karlsson and Stone challenging for a wildcard spot as soon as next season. That obviously requires canning Boucher and making a few clever roster moves (which implies replacing Dorion). If we trade Karlsson and a few other important players (like Hoffman or Brassard), however, we essentially commit to a full rebuild without having the necessary requirements (the picks Dorion threw away).
Ultimately building an actual contender requires good management, good coaching and the willingness and ability to pay for good players and good staff. Right now the Sens have none of these three requirements, but even if Dorion and Boucher were replaced by competent people the Sens won't be contending as long as Melnyk is here, rebuild or not. Even if the Sens committed to a full rebuild, brought in capable management, traded the current core (Karlsson, Stone, Hoffman etc), and by some miracle ended up with a core like the Leafs have now (pretty much the best case scenario for a full rebuild) - does anyone actually believe that Melnyk would be willing and capable to pay all the players you drafted plus those you bring in to compliment them/fill holes plus surround them with expensive staff, good coaching, the full works? Again, look at the Leafs: They didn't just draft a few superstars, they also pay an extensive management team with experience and different voices, they scout pro players in Europe to bring in guys like Zaitsev or Borgman to fill some holes so they only have to sign two instead of four over-the-hill UFAS, they actually identify veterans that fit the core reasonably well and are not completely useless - does anyone see Melnyk paying for all these things? Do we have any indication that Melnyk is willing or able to take the financial impact of not making the playoffs for a few years while you rebuild anyway? Because it sure doesn't sound like it. Realistically if your owner isn't up for a full rebuild you can forget about it.
We have one of the best players of this generation on the team - he's not that right now, but he can and will return to being at least a top 5 defenseman in the league. We have some of the makings of a supporting cast, dragged down by coaching, some deadweight, and a stupid management philosophy prefering 'intangibles' and 'experience' to speed and skill. I'd much rather try to re-tool around Karlsson than to rebuild and hope we luck into another dominant player.