Societal investment towards mechanization should be considered. Trying to tax equity of others will work in the short term but long term state investment allows for a base line to government revenue.
A 40 hour work week is not slavery. However it is an arbitrary line to decide that every person should work 40 hours a week.
Forcing people to compete with robots for mundane labour is a pointless endeavor. Structuring the economy so that everybody is skilled is not a good long term plan. Not everybody needs to be an engineer, doctor or scientist. Not is every person able to do the work required.