It's only a matter of your opinion that they should not have made it. It is a simple matter of fact that they did. Usually fact trumps opinion, but that's just my opinion.
Boston was the first time this team, this system went to the post-season. Pretty natural that they struggled initially. The fact that they went out in double OT of game 7 against the eventual two-time Cup Champs is more telling than what happened when they first got to the post-season. Again, this is just my opinion.
Anyhow, I don't see how the team is weaker. They lost Methot, true, but advanced stats indicate that Methot actually held Karlsson back (his Corsi and other stats were better with just about every other d-man). His ice time (which was 4th on the team, by the way) will be replaced by increasing ice time for Phaneuf, Claesson and others (Chabot, Harpur, Englund, Jaros). Outside of Phaneuf, it's unlikely that any of the others will make the team stronger, but at least the impact of Methot being gone will be partially offset. Up front I think the team -- once Brassard returns -- will be stronger because Neil, Kelly, Wingels will be replaced by the likes of Thompson, White, McCormick. A full season (hopefully) of McArthur.
And, of course, the ace up their sleeve: improved powerplay. Don't forget that early in the year they were concentrating on structure. I think Boucher said they won't get to work on the powerplay until December, and in the second half of the season they hardly ever had time for practice. This year they will start with a solid base and will be able to devote more time for the powerplay. Wherever Boucher's been his powerplay has always been good. No reason this would stop when we have Karlsson, Hoffman, Stone, Brassard and others.
Your "team is weaker" is weak.