Might I be the only one who doesn't have a problem with Riker staying on the Enterprise as first officer?
If he was happy with his position, why leave?
Perhaps the experience with the Borg in TBOBW was a little bit too much for him? He needs to make sure he is absolutely ready for the big chair.
Deanna. Maybe Riker hopes his relationship with her might develop into something more than shipmates. If he left to command some other ship she wouldn't be around. In TBOBW he says to Deanna that he has sacrificed a lot for his career meaning her, perhaps it's time to change that?
Starfleet asked him to keep an eye for Picard for a while after the Borg experience, who knows how messed up he might be?
It's a fair point. My issue with Will turning down promotion after BoBW isn't so much about why he'd want to. They've gone a long way by this point to demonstrate that this is a family, who all really feel at home where they are, & Will, more than any of them, has charged up the ranks to where he is. There's even later precedent to support that it was maybe even a little too convenient that he did so.
So now he's Picard's XO & it's been eye opening seeing what a leader really is. Lord knows Pressman couldn't have been much of a role model. That Riker's convictions about himself should be up in the air is understandable. I'd push forward, but I'm not him.
My issue is the flip side. It's not just about him, but about what the fleet needs. Even before Wolf 359, Picard makes it a point to tell Riker that Starfleet needs captains, especially now, & that he's ready. We have to assume that after the massacre, it needs a LOT more of them, badly. So badly maybe, that it's never even considered that Picard, of all people, should step away from his spot.
Shelby is right. Riker is clogging up the process. His current post isn't a weigh station. It's a position that trains new captains, ones coming up the ranks, who need that experience, like Shelby. It's maybe the best one... him squatting there like 16 years blockades a valuable avenue of advancement. Picard could mentor 3 or 4 others just like him in that time, which helps restore the fleet, excellently.
His choice, while understandable on a personal level is a hindrance to the fleet, & no matter what his personal proclivities are on the matter, he should also have some loyalty to the ideals of having a duty to serve. Some part of him should be there for the greater good, if he's a righteous man.
At a certain point, command must've just written him off, & his post, as a loss, & left it alone, because he did after all save everybody, & if this is how he wants to cash in that currency then so be it, but from their perspective, it's rather unfortunate. You're right that it does help to have a trusted guy there to watch post-Locutus Picard, but that wasn't really proven to be needed, & others could've done that too.