Manually sorting letters isn't actually the alternative here - what the US Postal Service was moving towards under DeJoy (and probably before him too) was sorti

Manually sorting letters isn't actually the alternative here - what the US Postal Service was moving towards under DeJoy (and probably before him too) was sorting the same letters using the same kind of automatic machines, but a smaller number of them reconfigured to sort into a larger number of different piles using parts from the scrapped machines. This reduces overall capacity, but should be more efficient so long as they don't actually need the extra capacity.