Can we use online video lectures to free university professors from lecturing for lower-division courses in mathematics? During the 2020-2021 Covid-19 season, universities and colleges around the world made most of their math classes online over Zoom and many professors have their recorded lectures.
Most math lectures usually don't involve in-class discussions and for the most of the time, professors dominates the teaching. Oftentimes, excellent recorded lectures can be much better than live lectures in mathematics and other theoretical sciences. I have seen many students commenting under certain YouTube videos that those recorded lectures are much better than their professors'.
I wonder if we could possibly use those recorded videos (there are a lot of such videos modulo-necessary-editing during Covid-19 season. Building a system like (https://nptel.ac.in/) should be feasible) to free university math professors from repeatedly giving lectures on lower-division math classes like calculus, linear algebra or differential equations (but at the same time, asking them to hold discussion sessions and office hours to have more interactions with students and of course, grading homework assignments and exams), so that they could have more time on their research and teaching classes that are  more relevant to their research? I believe this is particularly important for the early-career mathematicians. Many postdocs in math are assigned with lower division classes that are kind-of digressions to their research.
This seems to be a win-win strategy for both professors and students in terms of the mathematics. If this is good then maybe it should have been conducted already like https://nptel.ac.in/. What would be the potential challenges for doing so for higher education in United States and Europe?
Yes, you absolutely can!
The win-win is very unlikely though. Students won't see lower tuition or anything, and will have no opportunity to interact with anyone at the faculty level (so no letters of recommendation, no research jobs, etc). Professor lines will be closed on retirement (or through layoffs) because who needs them, we've got McGraw-Hill eLectures.