Then we come to the rub: if "don't show me adverts" suddenly made common tasks (checking email, using a search engine) cost a bit of money, how many people would go for that?

Quite a few. Hence people who pay for Netflix, YouTube, etc.

Certainly I don’t have adverts when I check my email, does Hotmail etc do adverts now? I’m sure they didn’t back in the early 00s.

Netflix is not a good counter argument, since lots of that content is not (legally/easily for casual users) available anywhere else even with ads.

YouTube premium might be a better counterargument—but I’m not sure how many people are buying it just for ad removal versus buying it to get access to additional games/movies/content. Is that data available? I’d be interested in checking it out if so!

Hard to tell, as those 2 specific services are currently monopolized by a company who refuses to offer those specific services ad-free

Email certainly isn’t monopolised! I can think of at least 3 mainstream search engines - although all advert driven