What is lost when academics teach to empty lecture halls?

Perhaps lectures just aren't that good... As to lecturers being able to "access to subtle signs, such as body language cues, revealing whether students were absorbing the material". Let's get real. There's certainly no way I can accurately judge what proportion of a 100-200 audience is understanding a concept from subtle clues. And even if a small minority haven't grasped a concept, are you really going to keep going over it until everyone understands it, however long that takes? Sure we can sprinkle a few interactive elements into lectures, but they remain am overwhelmingly didactic experience for one way transfer of knowledge. Perhaps we should just accept there's no particular benefit for students being present for lectures and instead focus on introducing properly interactive sessions into courses where attendance really benefits students.

@darwin59 "As to lecturers being able to "access to subtle signs, such as body language cues, revealing whether students were absorbing the material". Let's get real. There's certainly no way I can accurately judge what proportion of a 100-200 audience is understanding a concept from subtle clues." Not all lectures are to large classes, in fact class sizes naturally tend to reduce at 2nd year, 3rd year+ because students specialise more as their degree progresses. In my subject (Statistics) a third-year+ course might well have only a dozen, or fewer, students enrolled: It's perfectly possible - and it's also important - to judge how well one is going over in a class of such a size. For example, when one is discussing or trying to communicate a subtle concept, or even worse a slippery one! - and there are plenty of both in mathematical subjects - then losing the body language feedback from the room can indeed be a serious issue.

Education is all about adaptability and both students and professors need to utilize new technologies to find acceptable solutions. Online lectures/assigned readings can be followed up by using Q and A sessions after students have viewed the lecture. Q and A sessions can be made mandatory or use a credit system similar to those used by many certifications agencies. There are a wide variety of solutions out there and professors need to be more involved in utilizing those that best fit today's students not those from ten or twenty years ago.

I am reminded of series of scenes in a 1960s US teen movie where around 100 students gather in a large room for a lecture at the start of their degree. The next scene is the following lecture and has about 50 students in attendance with tape recorders on the empty chairs, and a bemused looking lecturer. The next scene of the next lecture has an even more bemused lecturer in the room by himself and every student chair with a tape recorder. The final scene in the series of the next lecture, has all student chairs again populated with tape recorders and a tape recorder on the lecturer’s desk playing a pre-recorded lecture! It was a very amusing cameo in the film. The point is that these concerns are not new and not related to Covid or newer technology. University students are adults who make decisions for their own individual reasons. I do my job in the way I judge best and each student does theirs in the way they judge best. And on the former, I would insist on clarity on whether my lecture was being live streamed, recorded for later asynchronous viewing, both, or neither. Then I would design and deliver accordingly since, plainly, the same lecture can’t be designed and delivered in the same way in each of those circumstances. Tailoring delivery of lectures to varying circumstances is part of doing my job the best I can.

“I was told to give the lecture anyway, because some might be watching it streamed,” and the university wants it recorded for future use when you leave, die or are made redundant...