It's not a strict limitation of the hardware as much as a product of changes in High Sierra. Namely, apfs being optimized for flash storage and the mitigations

It's not a strict limitation of the hardware as much as a product of changes in High Sierra. Namely, apfs being optimized for flash storage and the mitigations to critical Intel vulnerabilities that reduced syscall performance by 30% or more. The tl;dr is that if you want to breathe new life into those Macs, install Linux and run with mitigation=off or format to HFS+ and roll back to El Capitan or Sierra. I've got a macbook2,1 with an SSD on Snow Leopard, and it's still the most responsive, user-experience focused Mac I own.