Especially considering the mount of electronic waste that 400M of PCs represents. I think this should be one argument authorities should use against Microsoft when it makes all this hardware obsolete with a software update.

If there are software workarounds that can prolong the use of that hardware, even at the cost of performance, they should be liable (and maybe subsidized) to implement them.

That's a red herring. Win11 supports cpus with lots of bugs. Imagine what the errata document for the Snapdragon CPUs looks like.

Afaik it's mostly about TPM 2.0 and virtualization support.

If people are happy or stuck with their fully working machines, it should not be up to Microsoft to arbitrarily turn them into e-waste.

What’s the basis for requiring Microsoft to either/both: support them in Win11 or continue Win10 support beyond the timeline they published ages ago?

I do hope they do offer a practical path here, but I disagree that they should be able to be forced to do so.

Users who are happy with those machines can continue to use them as-agreed.

They don't have to keep supporting Windows 10 but they knowingly decided to cut support for 400M devices because of some arbitrary hardware features, and that's unreasonable imho.

This.

Spectre and meltdown, while academically intriguing, don't really matter in practice for consumer devices.

Every kernel I have booted has had mitigations=off.