I think something along the lines of a Pentium Pro or an ARM core. Pentium Pro had 5.5 million transistors. A modern CPU has about 1000x more, so about a thousand Pentium Pro-grade processors would fit in die like a modern 7770X.
I'd take that over my GPU any day.
The hard and expensive part is, obviously, memory, cache, and interconnect. The even harder part is software. And the less hard part I'm intentionally oversimplifying is power consumption.
If someone could wave a magic wand, and there were OS, app, compiler, video game, etc. support for both MIMD and current architectures, I think MIMD would take over overnight.
Most of what computers do is ridiculously parallel. From each browser tab getting an isolated CPU, to having a spreadsheet spread out among cores, to rendering fonts in a document.
However, given a universe with trillions of dollars invested in the status quo, a disruption would need some sort of rather complex pathway, with some niche markets, some growth strategy, etc. As someone pointed out, Intel tried with Phi and failed.
An example of MIMD system is Intel Xeon Phi, descended from Larrabee microarchitecture.
https://en.wikipedia.org/wiki/Larrabee_(microarchitecture)
Its x86 cores were based on the much simpler P54C Pentium
https://en.wikipedia.org/wiki/Xeon_Phi
That is the core from the generation before the Pentium Pro. Larrabee was supposed to be a GPU, wasn't good enough to compete at that, then they rebranded it as Xeon Phi but cancelled it a few years ago.