If you continue that thought experiment, you'd very quickly run into the issue that the way the 386 interfaces memory is hopelessly primitive and not a good match for running 1000s of cores in parallel.

A large reason why out of order speculative execution is needed for performance is to deal with the memory latencies that appear in such a system.

That gets you close to Larabee/Xeon Phi, although that was pentium based although amd64 and a vector engine were added and later products were Atom derived.