I suspect with the right message they could get very high conversion rate.
Come to think of it though I actually would send a donation if a large portion of it went to the professor, although 'to advance the field' and 'because it's my passion' as reasons for teaching do make for better classes.
I don't doubt that this has helped his profile with non-ML people - who might be in business positions to hire him as a consultant - but he was renowned as a machine learning expert before this.
Asking for donations solves this problem in a simple, frictionless way and may even be more effective than upfront payment. I think you're irrationally opposed on moral and ideological grounds rather than effectiveness, which is ultimately what matters.
Effectiveness without morality and ideology is just Sociopathism. To quote an old saying "The trains always ran on time during the Nazi regime".