Last I checked I'm paying an absurd markup, much larger than $15/month, on my internet access.
There is only minimal competition as it seems to be nearly impossible for other companies to lay fiber or cable, DSL is too slow, cellular coverage is spotty, and satellite suffers from excessive latency.
The problem doesn't seem to be lack of money paid by internet users, but where it goes.
I very much doubt the actual value of a typical ad-viewing user is actually $15/month. More like $5 maybe which becomes way more affordable and when you think about it, you're already paying like 10x that (if in the US) for internet access so another $5 for a spam-free experience doesn't seem too bad.
Maybe us software engineers with our awesome salaries can afford the $15/month for an ad-free web, but:
1. Many either can't afford this or won't see this as a need, because
2. Many people don't care about ads being there (see also: YouTube Red), so
3. Content generators will continue to post ads, which
4. Are easy to block if you're even mildly technically savvy