I definitely believe it would be possible by late 2019. If I'm not mistaken, AMD is releasing the successor to Raven Ridge in 2019, their mobile APU line. You'd be looking at a low power APU (the current Raven Ridge APU's range between 12 and 25 watts) with Ryzen 2 CPU cores and Navi GPU cores. Then all Sony would have to do is underclock it or disable some cores to reduce power consumption so they could use desktop GDDR5 instead of LPDDR4, and make it larger than Switch so it could have a bigger battery, and they'd have a system comfortably able to play PS4's library (after patching) at reduced resolutions with good battery life.
Apparently very few can see Raven Ridge, and even some who do refuse to acknowledge its exsistance. I was very shocked that Mr. Leadbetter did not bring up this possability, and even more so when he claimed AMD lacks the core technology to make it happen.
I still think Sony and AMD could take what has been learned over the last 6+ years, and significantly reduce the power consumption on a 7nm revision to the current PS4 APU. They have plenty of Memory choices that they could make to meet the needs of a PS4 at much lower energy draw. This would likely be the harder road, and may not pay off as much in the end, depending on where they are taking PS4, and when PS5 is coming out.
So, what is the other option? Exactly what you are talking about. Make a custom 7nm Ryzen Gen 2/Navi based chip, use either GDDR6 or HBM2. Now you have a device that blows the PS4 out of the water in the CPU department, and should be able to best it in the GPU department by quite a bit, all while falling in that ~10 watt teritory.
Now in theory, Sony should be making PS5 backwards compatible with PS4. If they are doing this and PS5 uses a Ryzen Gen 2/Navi based chipset, then they would already be working on making Ryzen Gen 2/Navi capable of running PS4 code without any modifications needed. Taking things even further, if they went this direction, then the PS Mobile device, should be ready to run PS5 titles at lower res/ lower frame rate, in very much the same way PS4 and PS4 Pro works right now. Effectively making it more of a PS5 Portable.
That is what I see looking off into the Road maps, and what technologies will be available by the end of 2019. Will Sony or Microsoft take a stab at it? Probably not, but I really hope they do. I think the market would go absolutely crazy at the idea of having Call of Duty, Battlefield, Grand Theft Auto, Assasin's Creed, Gran Turismo, Uncharted, The Last of Us, Halo, Gears, and Fozra/Horion all available on the go, with no compromise. But, that's just my thought's. At any rate, I look forward to what 7nm fabrication brings us, and where things go with Sony and Microsoft, whether it is Home, Mobile, both, or otherwise. It's all about gaming first hardware, and that is what I love.