That's the part I suppose it's hard to know especially without being directly involved with the team. Ultimately if there's a clash of players/management sometimes there's addition by subtraction even if the player in question is better than who replaces him. I also wonder if to some degree the trade wasn't the motivation Thornton needed to reach his potential. He wasn't really lighting it up that year as it was until after. Ultimately trading him might have been the best thing, Though the return could have been better/possibly helped win more than they did
Exactly, there's only so much we as fans can
know before it turns into what
we think we might know.
Also, I can't help but notice the rest of your statement is, word for word, almost entirely applicable to the situation between the Bruins and Seguin. But that's another discussion.
FWIW, I actually do remember hearing the "build around Bergeron, he's the future" stuff around the time of the trade (and I'm not a Boston fan nor plugged into their media market).
I can't find a direct quote or source right now, but that was the general consensus at the time. Even in his rookie year, the Bruins knew they had found something special with Bergeron. Chara may still have the C on his sweater, but Patrice Bergeron
is this franchise.
Anyone think his playoff rep played a role in this? Hadn't Boston bowed out earlier than expected after subpar performances from him his final years there?
I think it had a role. In the 7 seasons with Joe before the trade, the Bruins went to the playoffs 5 times, and got out of the first round once in 1998-99, when they lost 4-2 to Buffalo.
In 35 playoff games with the Bruins, Thornton went 6-12-18. Granted IIRC, that series in 04 he played with cracked/broken rib, he went scoreless with a -6.
In 125 playoff games with the Sharks he's 21-84-105.
There must be more to it than that. Why couldn't Thornton & Bergeron function as a one–two punch? Bergeron & Krejci did that. Bergeron has never been an offensive dynamo, less than Toews is, and Krejci led the playoffs in scoring twice when Boston went to the finals. According to this logic Krejci must be better for a teams success than Thornton. And, I don't know, perhaps he is?

I remember specifically in 2010 in the playoffs against Philly when Krejci got hurt, the whole Boston offense collapsed.
Remember though, Joe Thornton is 6 years older than Bergeron. They only played on the same team for about 75-80 games, and those were the first 75-80 NHL games of Bergeron's career. Bergeron was identified as something special as an 18 year old rookie, but they weren't relying on him in his first season,
I think he was the third line center that year. I don't think there's enough of a sample size to say that a Thornton-Bergeron tandem didn't/wouldn't/couldn't work.
Also, regarding Toews, he has put up more points than Bergeron, but think about the supporting cast he's had for pretty much his entire career: Kane, Sharp, Hossa, Saad, Panarin, etc...
Bergeron didn't really have that for the first few years. Krejci didn't get his shot until 07-08 (ironically IIRC, because Bergeron missed most of the season with a concussion), and then started to flourish in 08-09, also timed with the competitive run they went on for 5-6 seasons.
But yes, the big catalyst for the 2010 collapse was Krejci going down. They were decimated by injuries at that point, and an overachieving team ran out of gas.