Also, programming the necessary number of small MCUs to do this is incredibly tedious and difficult to manage
Only if you use the wrong tools and languages.
The ATmega328P only has 16 DMIPS of processing power
Even fewer available, if you continue to use delay () 
whereas a Raspberry Pi 2 has nearly 7000 DMIPS!
But very poor I/O drive capabilities, and is a bugger to program for real-time performance
Horses for courses.