Hronek is not a guy that you keep on your team if you want long term success. Just because he played first pair minutes for our dumpster fire teams does not make him a first pair defenseman.
Here's a list of the top paid defensemen in the NHL.
NHL Rankings
Paying Hronek 6 to 6.5 makes him the 25th to 35th highest paid defenseman in the league, and there is no way he belongs in the discussion for being a top 35 guy in the league. Hell, I don't think he belongs in the discussion for top 60. For reference, CHARLIE MACAVOY is #60 on the defense pay scale. #50 is Shea Theodore, #40 is old man Duncan Keith narrowly edging Colton Parayko.
There are maybe 2 guys in the 40 to 60 range that Hronek is better than; Danny DeKeyser and Anton Stralman.
Additionally, Hronek isn't so good that you want to make him untradeable. 6+ million for a guy makes him really hard to move. Impossible if you're dumb enough to give him a NMC when he's nowhere near a franchise cornerstone player.
Giving Hronek that kind of contract is how you end up perpetually bad, and we're trying to not be that.