Construction is already beyond the big wins in labor force though. Construction sites used to have armies of workers. Hundreds. Now they have perhaps a dozen for large skyscrapers, 5 for normal skyscrapers and one or two for houses.
On which construction sites have you been recently?
A year ago when I was working in construction, I remember for a standard home at least:
Planning (architects), demolition workers, excavation, foundation builders, bricklayers/concrete builders, plumbers, electricians, woodworkers, window workers, fireplace/chimney builders, painters, inner decoration guys, telecom/smarthome engineers. On one single home site. Oh, and add for at least the "big and dirty" parts of building a house at least two or three trucks with drivers.
But, of course, the companies did rarely work at the same time.
I see you point that the amount of labour involved in building projects has decreased however I disagree that no further labour decreases are possible. A typical house in the UK is currently build from bricks and wood with a tiled/slate roof and a concrete subfloor. The interior is made to look nice by application of plaster. A typical block of flats has a metal frame but the walls are still brick/breeze block (in the US cinder block I think). This requires several skilled workers commanding fairly significant wages (bricklayer, carpenter, roofer, plasterer) in addition to the plumber and electrician.
If houses were to move to a prefab model with e.g. plastic construction and standardised room sizes and plumbing and electrical connections between the rooms most of these skilled workers would find themselves redundant...
Fortunately the UK insurance industry inadvertently protects the building industry by making it difficult to insure so called 'non-traditional' constructions.
> If houses were to move to a prefab model with e.g. plastic construction and standardised room sizes and plumbing and electrical connections between the rooms most of these skilled workers would find themselves redundant...
Still, the prefab blocks have to be built by adequately skilled workers - gas, water and electricity at least will always require a certified worker simply due to the inherent risks of fire, explosions, water leaks and electric shocks.
This is true but a lot of labour intensive processes can be done off-site in a more controlled environment.
For instance, partial pre-assembly of mechanical/electrical fittings through to modular kitchen/bathrooms with almost everything done before it gets to site.
Of course you'll always need people to put all of these elements together but the numbers needed becomes less and less.
The certified worker who signs everything off used to do almost everything himself, we are now moving to a situation where this is almost all he does.