The mistakes Letang made on the Kuznetsov goals were not just timely errors or common gaffes that plague any top pairing defensemen, they were the kind of mistakes you never see made by NHL defensemen. Even the lowest of the low on the worst teams don't leave an entire side of the ice open with a player rushing down the ice. It is insane to see that play happen in game 5 of the playoffs. It would have been eye-opening to happen in the pre-season. His attempt to top it in game 6 may have failed, but it was still an equally devastating gaffe that would have driven a lesser name onto the first bus out of town. Unbelievable that he failed to learn from his mistake in game 5 too.
I appreciate what Letang has done here over his career, but you simply can't excuse those type of plays in the playoffs - with him being supposedly healthy. You can't have players with that potential on the roster in clutch situations. Letang has to be removed from the roster in the offseason - period. Its the 800 lb Elephant right now. If they fail to deal with it and move on, they'll fail to make the playoffs next year. Guaranteed. The room will never find their invincibility and confidence if #58 is present amongst them. Those guys might kiss his behind in public, but deep down they know hes the guy to blame for their failure to 3peat. (I think winning in 2017 without him is a big part of it too).
After Letang is gone, i'd get Olli Maatta the shortest leash heading into 2018-2019. He might yet turn his career around, but i'd keep all eyes on him to be sure. I am sure it wasn't a coincidence that his biggest brain farts happened with #58 on the ice, so maybe he looks better with Letang out of the lineup.
Beyond that, you've got a good roster set for next season. Whatever returns gained from trading Letang are just a bonus. Get a 3-4 D, ideally a good right-handed shot, and you're all set.