2019 is most likely, since it’s the 25th PS anniversary. 6 years is enough, last gen‘s 7 were unusually long already.
Why would "25th anniversary" be justification for new platform launch?
If anything, it is opportunity to offer "Special 25th anniversary edition" of PS4Pro with cosmetic details to attract PS4 owners.
Sony doesn't need temporary marketing gimmick to sell new platform, the platform should sell itself in launch year and after.
What does "Sony doesn't want to be the one reacting" actually MEAN when Sony depends on AMD to deliver core of product?
If you wait for CPU/memory process advance, you can define generation capabilities taking advantage of that leap in power.
If you don't wait, you spec performance at lower level, and non-APU/RAM parts will never benefit from re-spin price drop.