It strikes me that if you have different classes of people working on a problem then it doesn't necessarily matter what's more efficient on a per-person basis initially because some people will not be doing the work anyway.

For example, buying a power sander for 10 hours labour cost when you could just have sanded the table down in less than 10 hours manually.

It's not like there were infinite quantities of slaves or lower class workers, at some point someone is going to explore the idea of automation purely out of interest.

For what it's worth this is why I believe that inequality and some level of wealth at the top is useful and necessary. You want a class of people who can just sort of mess about as they see fit. Otherwise everyone is just scrambling to meet basic needs.