You and I remember Boston's playoffs very differently. Seids was at times paired with Chara to give Boston what was the best shutdown pairing in the playoffs, but he was often split away from Chara as well when the opposition had more then one line that needed to be shut down.
Boychuk can't replace that, he has neither the ability nor the savvy and of the guys you mentioned I'd only have Hamilton pegged as someone who can potentially backfill even Boychuks role, and he's still pretty raw to be expected to sure it all up on his lonesome.
Boston will need to be in someone of serious significance to fill that void. Everyone remembers the Krejci injury from the Flyers series, no one really remembers the fact that Seids was also a huge loss during that playoff run.
It's not that I remember the playoffs any different from you, I think we are just looking at it from two different perspectives. Whereas you seem to think of Seidenberg's injury as some sort of catastrophic loss, I prefer to look at it as a large obstacle that can be overcome by the Bruin's team defense. The Bruins experimented with splitting Sides and Chara up, but they were paired together more often than not, and I think that Boychuk and possibly even Hamilton may be able to fill that role by season's end.
I think you may be forgetting that Seidenberg missed 4 games in the PO's last year and was clearly not himself at either end of the ice after suffering some sort of knee/leg injury (groin?) early in the playoffs. Despite averaging nearly 26 minutes per game, he contributed 1 point and was only a plus 1 for the entire playoffs. The B's also lost Ference for 8 games. He was at 24:30 per game, and only contributed 2 assists (+2).
Both of those guys are "warriors" (no disrespect to the members of the armed forces), but contributed very little in the way of offense last season. Despite your assertion that Boychuk lacks the ability and savvy (not even sure what this means in this context?) to replace Sides, he actually outperformed him last season in the PO's, averaging 24 minutes per game, 7 pts (6g/1a), and was a plus 4. I would maintain that because of their lack of offensive output, they are more easily replaced in the B's system by guys like Boychuk and McQuaid stepping up, and hopefully their roles being filled by the youngsters.
I also think that with the play of Spooner, the B's have the ability to slide Kelly onto the 4th line and give them an even bigger role in the process, so that the B's are truly "rolling 4 lines" come playoff time, and wearing down the opposition, which should lessen the load on the D corps.
Seidenberg is definitely a big loss, but the B's have time to assess the damage and find out if internal replacements can do the job, before before making a costly move for a veteran #2 d-man.