Two ways to look at giving him a new contract at the end of this year:
1. If you don't, and he plays quite a bit better next year, he's going to cost you more than what you could sign him for this year.
2. If you do, and he doesn't play better, or gets worse, then you've overpaid and hurt your team for the next 2-3 years, until his contract dead money becomes manageable.
It comes down to what you see from him on the field, in practices, in film rooms, and even what other guys do. Lots of older quarterbacks in the league - Brees, Rivers, Rothliesbergher, Manning, Brady. Any one of those guys could tell teams they're only playing another year or two, Rothliesbergher says every year he's thinking of retiring, who knows when he'll be serious? Several new coaches around, maybe they want a young guy they can coach up their way.
Don't think for a minute the rest of the league views Dak like some here do - that he's average at best, won't get any better, gets ample coaching and can't win without loads of top talent around him. He's young, durable, a leader and experienced. That counts for a LOT with coaches, general managers and owners.
When a team pays Kirk Cousins, he of the 26-30-1 record after 2017, $28 million a year, if anybody thinks Dak will take $15-16 mil a year, they're delusional, even if he stays the same as this year.
So make your decision - pay him after this year and secure his services for a "reasonable" rate, or wait and either have to pay him top money or have a very good quarterback walk...