>>897279 (OP)
They are already making a comeback, several of the most powerful supercomputers in the world are made up of clusters of processors running on PCIe backplanes.
Exascalar system from several years ago:
https://www.youtube.com/watch?v=dojXiMBMNHo
The densities which the processors are packed means the entire system has to be bathed in fluorinert to cool it
Whats interesting about the exascalar systems is they are made up of Pezy SC processors, which are manycore processors running a MIPS-like ISA. The current generation chips have 2048 cores with 4 threads per core, with next generation planned to have 8096 cores.