Makes complete sense to me. With x86 CPUs and PC architecture all around, it would be super easy to support 3-4 iterations of hardware simultaneously. This doesn't make the original PS4 any less than it is, it just means that new games will play 720-1080p with low-mid settings of PC equivalents, and PS4K supports either 720p-1080p at high settings or 4K at low-mid settings.
COGS for a console that can match PS4 but at 4K native would be a bit higher than PS4 at launch, especially this year. They could probably keep the GPU part's cost down by focusing on adding components that increase resolution throughput (ROPS, RAM throughput) without adding more and more processing units like high end PC GPUs do. Put it on a Zen chipset to modestly increase CPU performance and you should be able to match settings at 4K.
If anything this extends the life of PS4. You could still play new games in 2022 that support 3 versions of PS4, they would just have lower settings. PC can do this with hundreds of possible configurations, surely a walled garden could do 3. Why would Sony do this if PS4 is selling well? For the same reason if PSX or PS2 could have done the same they would have to sustain their marketshare. or why Apple keeps releasing iPads.