Hey,okay, there's no need to worry!
Every season of South park has a long break in between it, that last around 5 months. This is for Matt and Trey, so the can chill out, go on holidays and spend it with their families etc.
The second part of season 16 will be airing in September ( On the 26th, i believe)
As for it continuing till season 20, this should still be happening.It was confirmed through comedy central, so no doubt they signed a contract with them.
Simple. It declined. South Park wastes so much time on the conflict/plot. Episodes that wasted so much time on the conflict was "Imaginationland" and "The Coon". After 2 episodes, you will start losing interest in the conflict. They waste so much time on it, they forget about jokes. South Park isn't even a sitcom anymore, its becoming more of a drama. Late season 9 was when South Park started to decline. Too much time being wasted on the plot. I remember when the conflicts were usually the boys being too scared to go to the woods because there was some monster but nowadays its always about some made up stupid thing and is so big it has to go through at least 3 episodes, I'm referring to the Coon. I'm hoping people here won't compare "Cartman gets an anal probe" to "imaginationland" or "the coon".