It is not generally considered unethical to set up in business, even though that will often 'demonetize' (reduce the income of) other actors in the same (or a related) line of business.
If you consider all monetization of web services to be unethical, then it is moot whether or not it demonetizes others.
As you are advocating the use of Ublock, it seems that you do not think the demonetization is ipso facto unethical.
Furthermore, the newcomer is offering an alternative form of monetization, and will presumably only succeed if that is regarded as preferable by some group of consumers.
While this seems mildly ironic, I'm not really seeing any argument, for this monetization of demonetization being unethical, that can be threaded through all these points.
In fact, does ethics even exist as a concept in business, other than when it's used to sell more products?
I agree advertising companies tend to be on the low ethical side of the spectrum, and often surf on the borderline of legal practices.
it doesn't in any way make those practice acceptable. but it doesn't grant another actor to step in and act in turn unethically.
I was thinking at first that any product that helps minimise ads and their diversion of attention is a win, but it's rather obvious that not any necessary evil is OK to exist so long as it fights another evil.
I find ublock origin doing its part without falling for the evil temptation. It's free and open source. I see little to no appeal to use a paid for ad blocker made by a developer with questionable ethics. even if that product happens to be superior. let's just make ublock better. and let's donate if we get value from it.