What is the rationale behind Microsoft saying that consumer should at least have 8th Gen CPU
You'll have to ask Microsoft ;)
MS is touting that there isn't much changed between Windows 10 and Windows 11 except for more reliance on TPM 2.0 or above and a couple of polished GUI and few upgrades.
So why the concern about whether your system will run Win11 or not? This is a serious question. I don't see anything in Win11 that's compelling. Meanwhile Win10 will be supported to at least 2025. I see no urgent reason to upgrade to Win11.
MS needs to address this issue pretty soon as in the present state of affairs, consumers do not have funds to go for a newer machine just to run the latest iteration of windows.
Again, this is an issue that consumers need to take up with Microsoft.
Keep in mind that other devices, most notably smartphones, are generally supported only for about three years. That's a lot shorter period than Microsoft provides for Windows.
BTW, I do agree that it seems odd that Microsoft drew the CPU line between 7th and 8th gen Intel. If they don't revisit that I wouldn't be surprised if some third party came up with a hack to override that check in the official Win11 installer. But that's pure speculation on my part. [OTOH perhaps there's some new hardware feature on 8th gen and newer CPUs that Win11 depends on.]
My comments are mine. I don't work for or speak for Lenovo.
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