No manager/CEO would say “hey, due to AI my workers get the same work done in 30h for which they needed 40h before - let’s cut their working hours down to 30h with same salary”.

Why not? The moment anyone offers this option, they get their pick of employees who will both churn less and be better rested for their work time, increasing productivity, whereas their greedy competitors get the people with no better options and burn them out. Despite people's despair, the current situation is extremely unstable.