There are rumors/leaks recently that are showing that Ryzen 4000 mobile APU's will be capable of PS4 levels of performance at just 15w. That's with 8 Ryzen CPU cores on 7nm next year.
If this is true, then Nvidia should be able to create a similar performing SOC soon that can do so under 10w with their tech.
The big question is how much would that cost next year? It should be reasonably doable all around by 2021 though. Definitely by 2022. By then we will be a year or two into a new gen with consoles around or above 10TF, so a 2TF handheld (while docked anyway), wouldn't be out of the question.