I would argue you don't need to upgrade at all. But if you know you want to and are going to anyway, two thoughts.
First, will you be able to be satisfied with the 17 Pro if you get it, and then the 18 Pro is released? If not, then just wait and get the 18 Pro. If you're already feeling the itch to upgrade and you have the 16 Pro, then it might not take long for you to say, "eh, maybe I should have the 18 Pro after all".
Second, if the rumors are true, and prices increase, the 17 Pro could become one of the last great Pro phones at that price point for awhile. And I don't see - again based solely on rumors - anything making the 18 Pro significantly better. I think anybody buying the 17 Pro now will be perfectly happy for years to come and might get the most bang for their buck when it comes to Pro phones going forward.