what if there was already a source of something equivalent to high speed SHA-256 ASIC. 
There are SHA-256 ASICs around, but they are basically I/O limited. They are fine for getting a hash of some given string, but in a Bitcoin miner you are incrementing a part of the input in a specific way. This can be done inside the chip, so you don't need a particularly high I/O with the outside world.