Yeah, it is likely that Querrey's ranking will drop next year, but maybe not by much. It depends whether he can clean up again in the '250s' and win some half-decent points at '500s.'
Unless he improves on his form from the second half of this season, he will struggle to be as dominant at the lower level (a little like Cilic), and is likely to lose his Memphis '500' title. However, the rest of his '500' results were terrible, so a few QFs would be useful.
His Slam performances haven't been bad, but, like Isner, he just can't get it together in the Masters events...too competitive for him. Unless he can go further in those (a couple of R3-QF), he will be capped at very bottom end of the top 20.
You look at his results this year, and you can say that his clutch ability to play the big points won him a fair share of matches. If it wasn't for a couple points here and there, his win-loss record would be drastically different. What I am trying to get at is, Sam better realize that he won't be a perennial top 20 player if he thinks he can pull off these tight 3, 5 setters every season. As time goes by, more and more people will read his game and make the proper adjustments. This offseason is the time to address that backhand, his volleys, and a stronger 2nd serve.
Otherwise, he's going to rely on clutch and luck again to survive tournaments. Those factors are also from confidence, he looks like he is missing that right now.