But then you realize that the validation of one block must complete before starting the validation of the next, since otherwise you have lost the required ordering of output spends that was provided by the utxo store. So you are back to linear validation (one block after the other). At which point the benefit that arises from the high cost storing and then reading each block is wiped out by the fact that validation has not been parallelized, and it looks pretty much like what you started with.