Our employers would suddenly expect us to spend that time working for them.
More realistically: it wouldn't be sudden, but we'd still get to the same result eventually, maybe in a couple decades.
https://ourworldindata.org/working-hours#working-hours-per-w...
I see no reason why longer days would reverse the trend. Most countries have legal limits on hours/week, and these would have to be raised, which would be immensely unpopular.
For example, prior to the industrial revolution, people worked roughly 1,440 hours a year[1]. During the industrial revolution, that shot up past 3,200 hours of working a year.
Today, in the US, the number of hours worked per year has stayed relatively stable since the 1970's. People work about 1,800 hours a year which, while lower than during the industrial revolution, is still more than people historically did in the past.
[1] https://groups.csail.mit.edu/mac/users/rauch/worktime/hours_...
Makes me wonder how metabolisms would change. Dining, working out, other biological activities- how would they also be impacted?