I guess we can evaluate this claim by looking at profit margins. In industries with thin margins (eg., retail often has 1-2%) then there is significant competitive pressure. There we would not see "spare labour" because it would be competed-away.

In industries with large margins, this indicates weak competitive pressure, ie., a weak incentive signal to "compete away" this ineffective labour.

I imagine that, upon this analysis, we'd find it holds up. ie., the observation that there are lots of people doing very little productive work correlates well with weak competitive pressures and hence high margins.

That also explains why, institutionally, there's no internal pressure either. If margins are good, no one rocks the boat.