The real environmental cost of ads is when people actually buy the useless shit being advertised. Also, you definitely have a strange attitude toward mining cryptocoins client-side and are engaging in many assumptions and false notions (that 100% of CPU will be used, that ad-tracking doesn't use just as many cycles, that it's not opt-in, etc.)
It's actually quite simple, any website that makes my laptop fans spin up gets closed out. None of the websites with ads trigger this behavior. The cryptominers do. If you look at actual CPU usage you will notice the obvious difference.
This makes sense, since the entire idea of crypto currency mining is to use CPU cycles to mine the crypto currency. While ads is just some basic tracking and graphics.