that's not true, i want ARM and soon RISCV boards
Another thing although speculative: Windows 11's move to require UEFI/x64/SecureBoot could be prep for AMD and Intel to completely drop a ton of legacy support (16bit etc.) in the chips. I'd give it about 20% chance of happening, but I definitely wouldn't rule it out given you can emulate a 386 easier than you virtualize one.
Like the 80386SX? 80376?
On the topic of downleveling... based on the research I've done (think writing a UEFI capable kernel for FreeDOS) it's dubious at best to do that because the real mode core wouldn't know about the local APIC and would mess with the IO APIC when you really really don't want it to. So is it possible? Potentially? Would anyone with any shred of sanity recommend it? No not in my opinion. There is no valid reason to do that.
china? you bring politics too?
who are you?
The whole selling point of x86-64 was that it was an extension. You didn't need to use the extra registers, the long address space, etc. except where it actually was useful. I'd be unsurprised if a lot of x86-64 binaries have plenty of traditional 32- and 16-bit instructions. Maybe there's a handful of "can never sensibly be executed in a 64-bit OS running normal well-behaved software" flows you can nibble off at the edges (stuff related to long-abandoned 286 protected modes?)
If you go much further, you sacrifice the key selling point of buying an x86-64 CPU: the ability to run your closed-source line of business software and propriatery games. Then you've basically got the software value proposition of one of those arcane POWER or RISC-V desktops, or an ARM Chromebook, without the unique selling points of either.
I'd expect the portion of the die responsible for decoding 16- and 32-bit instructions has more or less stabilized over the years, and just gets copy-pasted-shrunk across generations. MOV AX,[1234:5678] is pretty much unchanged in 40 years, so I doubt there's a hot breakthrough in how to decompose it to micro-ops.
The transition I could imagine would be a big-little style thing: a CPU that was, say, eight x86-64 cores and eight RISC-V or ARM. Over time, the ratio skews, until the x86-64 cores are a co-processor you can install seperately if you still need them.