The Fed tracks this: https://fred.stlouisfed.org/graph/?g=tjto

Unit cost on labor has increased at a more or less steady pace this whole time. Ergo, it's not so much that labor is decreasing as other things are increasing faster.

It's hard to argue that technology is increasing labor productivity an order of magnitude faster than it was in the 50s. It's more likely something else in the dataset (returns on capital/rent) is exploding in value.