There is no problem with my reasoning, because the world has settled on 35-40 hrs/week as being about the right balance. 100 hours doesn't work; my conjecture is that neither does 10.

There's a problem with that reasoning, because that number exists only because of Judeo-Christian traditions about the days that begin and end a week, the fact that 8 is a third of 24, the fact that the US had an active labor movement in the early 20th century, and the fact that US-style capitalism has been very influential around the world (largely for military reasons.)

Basing a chain of reasoning on the premiss that the status quo is the result of careful calibration over time is abandoning reason at the outset.

100 hours worked just fine, for owners.

A few questions about your larger assertion:

1) Could a 2-day a week dentist have a longer career than a 4-day dentist, ultimately leading to a higher saturation of experienced dentists?

2) Do you think that between a 2-day and a 4-day dentist of equivalent experience that one would perform better on an average workday than the other, or would they be equally affected by fatigue?

3) Do you agree that time for reflection and study are a requirement for skill-building, and do you think that there's any evidence pointing to an ideal ratio of reflection to practice that wouldn't advantage the 2-day dentist over the 4-day one?

Only if you're cynic to the core. If you follow that argument through (many do), then you would be okay with engaging in highly unethical (yet legal) behaviour.

> the world has settled on 35-40 hrs/week as being about the right balance.

If the criterion for "balance" is actually squeezing the most of your employees. Which works wonderfully mid term (1-2 years), but I wouldn't be so sure about long term (10-20 years), where time devoted to directed learning could compound to a significant a difference.

More importantly, though, if maximising your employer's benefits is your goal in life, something must be deeply wrong with you.

They say yes, you say no. I see no reason to believe your version over theirs. (no, "everyone does it" is neither a proof, nor a reason, it is "that's the way we did it yesterday, that's the way we will do it in the future, why should we change?", the motto of almost every doomed enterprise)

The naturalistic fallacy.