Ryzen processors have 80MB of L3 cache! IIRC that may have been twice what was needed to run the colorized versions of NeXT.
A lot of "man the old days were all we needed" recollections of Windows and DOS forget how crappy those OSes were...
But NeXT? That was basically a modern OS, with a bit less anti-aliasing. XWindow systems STILL look horrible compared to it.
It fits in L3 cache. 5-10 NEXT hard disks fit in RAM!
It had a web 1.0 browser, TCPIP networking, all the productivity apps. It had DOOM.
Sure Mach the microkernel was buggy and leaked memory, but kernels... there's lots of kernels now.
I think it would be a great project to start with the NextOS and carefully rebuild it up with good security and multitasking.
It would be fun to run NeXTSTEP on bare metal with RAMdisks (spare a processor to do the writes to SSD), and compare with other systems, see if it feels ludicrously fast or not.