It's pretty clear where the show took a massive nosedive and that Trey is no longer interested in writing for South Park, but he also refuses to hand over the reigns to somebody else. Not to mention the viewership. Whoever is funding Trey is mentally fucking retarded.

This chart over here explains it all. I haven't seen such a pronounced downward spiral and "I quit" message since never. Trey made it obvious numerous times with episodes such as in S15 with "You're Getting Old" 2parter, S16 with episodes such "I Should Have Never Gone Ziplining" , "Sarcastaball", "Raising the Bar", "Insecurity", "Going Native" that he quit and the S14 Coon & Friends was the finale to the show.
The question is what the hell happened? he got bored or did Comedy Central screw with him? Why would CC allow him to reduce the episode number even further rather than trying to raise the number to 26 per season like it's normal for a TV Show??