"We...ell... there are people who have poor (or no) eyesight. My guess is that they would tend to trust that specific plugin - or at least grudgingly accept it"
True. But that doesn't mean everyone else also has to trust that specific plugin and have it running in the background every time they visit a site. You're correct that perfect security isn't possible and that at some point we have to trust something, but there's a huge difference between allowing some exceptions where necessary, and expecting every user to blindly load hundreds of external resources just in case a tiny proportion of people might need them at some point.
That's why script and ad blockers are essential. Yes, you need to make some exceptions and so can't eliminate all possible vulnerabilities. But a big problem with the internet in its current state is that the default expectation is that everyone will always trust and run everything regardless of whether there is any reason to do so. Until the opposite is true and the default is to access resources only when actually needed, blockers are necessary to enforce that state.