You could make that argument, but nobody does because we've had three up-three down at higher levels since 1973/74 and there's never going to be much of an appetite to change back to an old system that doesn't offer any obvious benefits.
Between tier 4 and non-league since 1973 we've gone from a re-election process to one up-one down automatically (together with a stadium requirement rule that prevented numerous clubs from being promoted or relegated) to two up-two down and also a change in the structure of the play offs at 5th tier level.
Additionally, it's the same two-up and play off system in tier 6 North and tier 6 South and maybe the same in some leagues lower down. They won't be able to increase without relegating six clubs, which would be ridiculous.
Great if 3 up-3 down happens, but removing my Notts scarf for a moment, I can see why this idea has been rejected over and over again and will probably continue to be so.