Why do you doubt it? I've read that extension creators are constantly bombarded with offers to package just a little malware in their extension; surely the same is true of ads. If you're just a hobbyist developer—and that's where the best extensions come from—surely it's too much to expect that you'll resist that pressure forever.
I assume that everyone who makes useful extensions like this does so out of good will. But I also assume that it is possible for a good person to tire of maintaining a piece of software, and want to pass on control, and to allow themselves to see the upside of an appealing and lucrative offer and believe that the worst won't come. And I can't blame a person who does this—they certainly don't owe me anything.
1. The creator of the VLC media player was apparently offered "tens of millions" of euros to insert ads. Thus, if your software is sufficiently widely used, that is the kind of temptation you may eventually face.
2. He said no. So there do exist people who will resist such temptation.
Citation: https://news.ycombinator.com/item?id=15372048 and the link chain therefrom
So there is always a significant risk even the author declines all the offers for cashing out.
But again, your awesome extension in which you poured so many hours is not the problem, it's just affected by the problem.