AMP is something websites opt into, absolutely. However opting out has a major potential to hurt your placement on search results. AMP is also limited in what kinda of ad formats it can serve. Want to take a guess at which work best? Google Adsense. Most other forms of advertisement just aren’t compatible, which can directly effect ad revenue.

Edit: Just realized I previously said “no revenue” which is incorrect because it can still produce revenue. So while “no revenue” is absolutely possible, it isn’t guaranteed. That’s what I get for using generalizations though.