>The top end Pentium 4 had a TDP of 115W vs. 170W for the current top end Ryzen 9000 and even worse for current Intel.

You can't compare CPUs based on TDP; it's an almost entirely useless measurement. The only thing it's good for is making sure you have a sufficient heatsink and cooling system, because it tells you only the peak power consumption of the chip. No one runs their CPUs flat-out all the time unless it's some kind of data center or something; we're talking about PCs here.

What's important is idle CPU power consumption, and that's significantly better these days.

>older phones or tablets with <5W CPUs, so why do those go out of support so fast?

That's an entirely different situation because of the closed and vendor-controlled nature of those systems. They're not PCs; they're basically appliances. It's a shitty situation, but there's not much people can do about it, though many have tried (CyanogenMod, GrapheneOS, etc.).

>Plenty of others who don't even care about 4k

Not everyone cares about 4k, it's true (personally I like it but it's not that much better than 1080p). But if you can't tell the difference between 1080p and an NTSC TV, you're blind.

>1080p TVs and even some 720p TVs are still sold new

Yes, as I said before, we're seeing diminishing returns. (Or should I say "diminishing discernable improvements"?)

Also, the 720p stuff is only in very small (relatively) screens. You're not going to find a 75" TV with 720p or even 1080p; those are all 4k. The low-res stuff is relegated to very small budget models where it's really pointless to have such high resolution.