The issue is that an opt in solution might make the system unviable in the first place, which reduces utility for everyone. Of course, opt out should be simple,

The issue is that an opt in solution might make the system unviable in the first place, which reduces utility for everyone. Of course, opt out should be simple, but if the majority of people don't care and the data is anonymised properly, I don't really see the issue since this is used to further the sciences.