While Sale and Price are certainly different people, it's not a "flawed assumption" that Sale wlll decline in the near future. It's an extremely likely reality with the only question being "when".
Not just when, but bu how much.
The evidence is in nearly ever pitcher ever.
There are actually quite a few pitchers who do very well after 31 and some that do even better than before 31. Some even do very well into their late 30's.
Really, with Sale, if you want him back after 2019, you have to be prepared that he will start to decline in 2020. It's not the type of contract DD should enter under complete denial that Sale will last for 7 years, which I doubt any GM has ever had that denial in a long term deal. If they did, the deals would be structured differently.
A slight decline from being the best may not be all that bad. A decline like Price's would not be good.
4-5 good years? Define good. He isn't not going to be a 6 fWAR pitcher at age 35. And at some point in his next deal, he is gong to be a heavily paid mediocrity. really, what it comes down to is, will the good years be enough to justify the bad ones?.
Would this be 4-5 good years?
Age WAR
31 6.5
32 6.0
33 5.5
34 5.0
35 4.5
36 3.7
37 3.0