I have a feeling that it is a hard argument to argue for. It is remember to note that supervisions are very Oxbridge-specific and most other universities do not have them making us more privileged. If you think about Engineering lectures for 350 people for 4 hours a week it only takes 4 hours of academics time per week. If you condense these to 1h session per week per 2 people you end up with a much higher total cost of 175 lecturer-hours per week.
I agree that if a lecture is simply giving away knowledge without any critical thinking/analysis component then it might as well be replaced with a recording. On the other hand, if a lecture is a discussion and involves heavy participation then watching it online strips it of some value.