I doubt it. Even with universal ad blocking on their platforms, iPhones are the minority worldwide and Macs even more so.

Websites would just make sites painful to use for Apple users and force them into exceptionally expensive subscriptions (which most people won’t pay for) or onto cheap devices that allow endless ads.

They could obviously forbid apps to show ads and implement a powerful, default-enabled adblocker into Safari. But why would they? The outfall would likely see many important developers pull their apps at least temporarily. Of the Top 10 Downloaded Apps of 2021 (https://www.macrumors.com/2021/12/02/apple-most-downloaded-a...) only a single one doesn't have ads. This could be Pyrrhic victory for Apple. It would most likely also seal the deal on sideloading regulation.

Yes, but let's not get over excited.

I'm fed up with their ads to subscribe to their services in macOS/iOS: the App Store, on Apple Music, Apple Arcade, Fitness+... I'm getting tired of clicking "no thank you".

Don’t confuse advertising with tracking. I believe that the true advertising business and the people who really understand marketing is going to have a lot of success in the coming years.

The business that will hurt are those who only know how to click around in Google AdWords or Facebooks Ads. Those business but all their eggs in a rather small basket. Facebook isn’t going away anytime soon, but it will fail more quickly that some expect. If 50% or more of your business is coming from Facebook, start making plans for the future now.

Apple has the power to extort massive amounts from the advertisement industry. Case in point: Google.

I think you are under impression that ad industry depends on tracking of individuals. That's incorrect. It just a tad less efficient without tracking.

Ads were around for hundreds of years before Apple even existed. The advertising industry will survive just fine.