I think Keith could rebound in a big way with proper usage and toi. He still skates really well, and reads the game well. He just needs to be put back into his element: he needs to be able to take risks and trust his partner defensively.
He also needs forwards who can find the soft areas to accept passes in the neutral zone. When Dunc is pressured to play a certain style or constrict his style, he forces plays. Usually it ends bad and he gets flustered.
Ultimately, i think he slightly improves... All things considered (mostly regarding Qs decisions). But if he bounces back, he could have another good few years. If not, he's going to trend downward from here on out. Just my opinion.
I think crux of Duncs issues from the Nashville series in 2017 until now are his own. He's making bad decisions in moving the puck. His coverage is shoddy because it looks like he's trying to over-cover for his partner instead of playing his gap, which means that the goalie has to compensate for 2 guys out of position instead of 1. Lack of D from his partners complicates the issue, plus the FWDs not making an outlet pass available--as you mentioned.
A big thing that would help Keith, aside form Q stopping his Wile E Coyote-level exercises in futility with this deployment of all players, is to focus on his own. Focus on his coverage and not try to do too much--especially if Crawford is back. Crawford has more than proven that he can bail out bad defense since his bad 2012 year so let Crawford handle a rookie/journeyman misplaying it and Keith should stay tight to his assignment. In essence--he shouldn't try to do too much and be the hero. If he keeps it simple on the ice--covers his man tighter than the anatomy of waterfowl, and lets the goalie compensate for the lack of his partner it would help his game a lot.
Conditioning-wise I'm not worried about Keith. He's a lot like Hossa or Kane in that regard--he's always ready physically for the opening puck drop.
I think the most major issue with Keith is Q. If Q alters his gameplan to fit the players we got, or we get a coach who's gameplan fits the players we got I think we see him rebound very well. But right now Q's playing Keith like it's 2010, 2013, or the 2015 playoffs...and that's not what Keith is anymore. Thus I think we see a rebound to a level where he's not a liability, but not stalwart, either.
A strong stay-at-home D partner would help him a ton, and that's why Joker should not play with him--Joker's game is too similar They both drive transition and are both the O-biased part of the paring. IMHO Joker and Keith paired would be a lot like Leddy and Keith paired.