PCs are already modular.
But if Intel actually wants to make it better they'd:
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stop changing sockets every generation or two (AMD still supports AM4 with CPUs they're selling right now, and seemingly only change socket when a new RAM standard comes out). To make matters worse, whenever Intel does use the same socket across multiple generations, it's only generations where there's a minor performance bump (or a performance regression)
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Refuse to certify non-standard OEM motherboards