My son's previous school is switching back to standard scheduling and is not replacing teachers who left. Classroom sizes are restricted so in block you needed more teachers. He had 3 classes average 30 each = 90 students. This year he has 5 classes average 25 each = 125 students. Block needed more teachers. Even if at capacity of 35 per class, block would be 105 students, traditional would be 175 students. In equally sized schools, block would need more teachers. NOTE: This year he has 1 class with 34 students unless they move some out. He also has classes with less than 20.
I agree that teachers who are flexible, fine with creative teaching & lesson planning would have no issue with shifting. There are teachers who have taught the same plans for years and now have to figure out a compressed lesson plan. At my son's school they did way more "meetings" than normal for teachers to share ideas how to shrink the curriculum. I get there are the same minutes overall but it has to be taught in a way to keep students engaged for a period of time that has never been normal for them. You have to change the lesson plan to keep them tuned in. This was harder for some.
During pandemic the huge plus was this way less students and teachers were interacting each day. Outbreaks were common but tended to stay contained to less classrooms. Luckily son was history and other than kids being oblivious to basic history/geography concepts his was a fresh subject each semester for students.