Wouldn't it be kind of silly NOT to accept a promotion? Especially since you have declined several in the past just so you could be in a position to captain the ship you are now being offered?
That's not why Riker turned down those promotions. He wasn't waiting for Picard to leave, he was making the most of the opportunity to serve
under Picard and learn from him. It wasn't the ship that was his motivation, it was the man. And the crew, the family. He stayed on in "Best of Both Worlds" because (like scripter Michael Piller, who was facing the same decision about whether to leave TNG) he was happy where he was and realized it would be foolish to pass up personal happiness out of some arbitrary pursuit of career growth.
And there's no question in my mind that Riker is very happy on
Titan. The crew and the mission are exactly what he wants them to be, a special kind of command opportunity that he wouldn't get if he ran the so-called "flagship" of the fleet (a label that has no real meaning beyond the symbolic) and had to be doing routine diplomatic and defense missions all the time. Sure, it'd be a step up in prestige, theoretically, but at the cost of personal contentment. And Riker decided long ago that he wouldn't make that kind of choice.
Even from a career-advancement standpoint, it doesn't make sense for Riker to go back to the
Enterprise. Why? Because Picard has defined the captaincy of the E for nearly two decades. If Riker succeeded him in that post, he'd still be in Picard's shadow. He'd no longer be defining his own path, leaving his own distinctive mark as the pioneering captain at the vanguard of the glamorous
Luna-class deep-space explorer fleet. He can make more of a name for himself where he is.
If Picard retired or otherwise became unavailable, I'm sure Starfleet would assign another veteran captain, someone with decades of experience at starship command -- most likely someone we've never heard of, or at least someone we've seen little of, because our focus as viewers has been on this particular ship instead of Starfleet as a whole. (Is Tryla Scott still around?)