Right now there are enough people willing to bend their back in the sun for hours that their salaries are much lower than these of engineers. Do you think that for some reason supply of these people will drop with higher wages and much lower employment opportunities in office jobs? I highly doubt it.

My argument is not that those people’s salaries will go up until overtaking the engineers’.

It’s the opposite, that the value of office/intellectual work will tank, while manual work remains stable. Lower barrier of entry for intelectual work if a position even needs to be covered, work conditions much more comfortable.