If the best argument in favor of HTTPS everywhere is that it will prevent your ISP from showing you ads, the movement is doomed.
If I ran a website with ads, and someone was stripping those ads to replace with their own ads, I'd be annoyed. I'd be amazed if that's something Google would tolerate. We've seen plenty of stories from people saying "Google closed my ad account and froze all my money!!!" so I hope they do that to this ISP and or the company serving the ads.
But, now they've seen their ISP tampering those people might switch on encryption for their email, and everything else.
For what it's worth I was grumpy in those situations too. I wrote polite letters. Where possible I opted out.
But your point - this kind of this happens all the time, and has been going on for years, and noone is doing anything to stop it even though it's wrong - is taken.