I think the major difference is that, if you're in this regime where you're getting diminishing returns on increased pay, for the employer it can be a lot more

I think the major difference is that, if you're in this regime where you're getting diminishing returns on increased pay, for the employer it can be a lot more cost-effective to improve working conditions. Trucking companies paying drivers 30-50K/year are probably not in this situation; software companies got there a long time ago for engineers.