I don't think there will ever be PS4K games that don't run on a PS4.
AFIAK, the PS3 is currently still manufactured (I can pick a new one up from Argos for £179). Regardless of anything else at all, I'll be pretty damn certain the PS4K will still be manufactured and sold for some time after the PS5's release. I think the PS4 will cease production before PS5's release. Read in to that what you will about PS4K not running on PS4...
PS4/K games will run on all newer PlayStations going forward.
Hopefully yes. Even without PS4K no one was expecting Sony to radically change the hardware enough to seriously effect BC. Hell, we all thought that was part of the reason for partnering with AMD in the first place.
Nothing is going to require a dev to make a PS5 game run on older hardware unless they want to.
Maybe not. Sony might put a temporary certification clause for PS5 in the same way they are now for PS4K, they might not. Ultimately what machines they support is a question for individual developers.
However, from Sony's and the consumers point of view. When PS5 hits PS4 owners have three legitimate choices:
- Super Chaep - Buy a secondhand PS4K
- Cheap - Buy a brand new PS4K
- Normal - Buy a brand new PS5
Whatever happens the PS4 userbase will naturally start shrinking compared to these two platforms. After all, the PS3 userbase isn't as strong today as it was 3 years ago, is it?
As a developer, where are you going to concentrate your efforts? PS4+, PS4K+ or PS5 exclusive? I'm sure the answer will be different for individual dev teams, but I'll happily place bets that PS4+ is the least popular option, and in the first couple of years PS5 exclusive would be rarities.
Remember 2016, with the release of first party games like Uncharted 4, is being hailed as the first real year of PS4. It's taken 3 years for PS4 development to get this far...