It's actually reasonable for sites to be able to estimate the proportion of their population who block their analytics by looking at say, the proportion of signups or conversions or sales or whatever that come from 'untracked' sessions. But that is confounded by the fact that the population who uses ad and script blockers is not necessarily similar in behavior to the population who don't.
If 2% of signups to my newsletter come from sessions that don't show up in google analytics, does that mean 2% of my site traffic is using an ad blocker, or are they actually 10% of my site traffic - but those users are just 5 times less likely to give me their email address?
It's like missing a test case that covers 15% of your user base.