My major issue with ads is that they're slowly obliterating the consumer surplus of the internet.
This is mostly anecdotal, but it seems much harder to find useful information on an internet search today than it was, say, 10 years ago. Any given search is polluted by a series of ad-laden pages with the absolute minimum information content required to get them to rank for a given search. There may be more useful information on the internet today than the 2011 internet, but it seems like it's increasingly crowded out by less useful information with better SEO.
It's gotten bad enough that I now use google image search as my default search, because that gives me a way to preview the content that will be displayed and quickly scan the results, and skip past the junk that google will feed me.
I suspect there's some sort of mechanism at work where spending more effort on SEO has a higher return than effort spent on creating useful information, so low information, highly optimized pages tend to outcompete more information heavy ones, and the modal webpage becomes one that is only barely more useful than it is annoying. I can imagine that preventing ad-based business models could lead to a better equilibrium (especially as high-value content is increasingly either created for free or put behind paywalls anyway).
Example: if I search "most reliable car", I will get a series of low-value pages, like this: https://www.topgear.com/car-news/list/here-are-17-cars-thatll-do-300000-miles#1 . Some are like this (basically useless), some are quoting snippets of a consumer reports or JD power article. Consumer Reports itself doesn't show up until the second page. And one of the most useful sources for reliability information, http://www.dashboard-light.com/, doesn't seem to show up at all.