Should they have won more? Yes. But other great Bruins cores never won anything (Bourque, Neely, Oates, O'Reilly, Middleton, Park), so the fact that they got their name engraved on the cup once is a blessing.
I've softened up on their legacy the closer we get to the door slamming shut on them. There are countless instances where if Daugavins and Kelly hit those open nets, or if Seids doesn't snap his ACL and if they don't hit the post 30 times behind Carey Price in 2014, or if Kelly Sutherland calls a damn hook and/or Marchand doesn't peel off his trailer in 2019, they win more cups. I think if not for COVID they have a very good chance of winning in 2020 as well. But...it goes the other way as well. If Thomas doesn't make a sprawling save on Gionta on the 2-on-1 in game 5, if the Habs pooch a goal in OT of game 7, if the refs in game 7 against Tampa don't put the whistles away, etc. then they have no cups and are a total failure that gets blown up in 2011. So it sucks that they didn't win more than one cup (you can't tell me the Kings were a better team than the Bruins over the years 2010-2015, but they somehow got two cups to our one), but they got one and that memory will stay with me forever as something to cherish.
The other part of the question of legacy is like what do you mean when you ask that - when you look at the main core of the team over the 10 year run - Bergy. Krejci, Chara, Marchand, Rask - is it a judgment on those players if they win or don't win in the postseason? I think the failings are more on management than on the players. Seguin trade, 2015 draft, Boychuk trade, signing Belesky and Backes, etc. I don't think it's fair to call the players' status as championship material into question because management set them up to fail on occasions.
Regardless, Bergy, Chara, and Krejci are faultless. All three should have their jerseys retired. Rask is a complicated case. People naturally blame the goalie when things go sideways, for right or (more often) for wrong. There were times when Tuukka could've given us more and had bad things happen at the worst possible times. But in both 2013 and 2019 he was playing at a Conn Smythe level and had a tremendous run until his natural bad luck came out and his season ended with clunkers in each case (2019 was especially tragic considering he was the unanimous Conn Smythe if they won that and it would've turned his whole legacy around).
Marchand is the one whose legacy suffers the most from the lack of any success after 2011. His rookie year he scores 2 goals in game 7 to win the cup, you'd think he's all gravy from there, but he's made a habit of not having great playoff runs, and that line change in 2019 pretty much singlehandedly altered his entire legacy.
There's lots of factors in winning and losing championships - player effort, coaching decisions, roster construction from management, puck luck, officiating, health, etc. All of these things influenced our playoff failures in various ways, but 2019 game 7 was the one time where I'd say a boneheaded decision a player made on the ice sealed our fate. And it's not a good look on Brad. But as time goes by, we'll suppress the negative memories and remember him as the guy who scored two goals in 2011 game 7. We just need time to heal. (I don't blame Daugavins the same way because he was a crappy AHL depth guy who was only in the lineup because Campbell broke his leg and Claude didn't want to have Soderberg make his NHL debut in the finals. It was an incredibly dumb play that I'm always going to be mad about but I don't hold it against him in the same way I hold 2019 against Marchand because Brad should know better).