That's definitely the way I would approach it.
If things work out and the team has success in 2019? Great! Pay everyone! Lock them all up to contract extensions. After all, they would have earned it.
But, by the same token, if things don't work out, nobody should be extended, because nobody has earned it.
And, if you're making a big change at the top of your coaching ladder, you shouldn't be locked into long term contracts who might not fit with what your new coach wants to do. Therefore, it's best to retain that flexibility to make your decision one way or the other, without being unnecessarily tied to it before you have to.
I'm of the belief that Jerry Jones has a plan in place for 2020, and that plan includes Garrett getting an extension, Sean Payton, Lincoln Riley, and Kris Richard. I feel that one of those four people will be this team's head coach for 2020.