I don't mix to the lowest common denominator but many do and many so called Pro's even mix on macbook speakers. That is no joke and a sad reality.
In my point of view, people don't realize how good is his setup because they aren't shearching for a good audio setup, most setup you can use a mic and then make a EQ correction profile, plus not too loud listening level and you can work pretty well.
But some people don't have emotions attached to, for example new speakers. Even if the new speakers have better performance they need to find a way to translate his emotions into these speakers that give something nice in his life
But most people mix with cheap stuff.. something at 200$ - 400$ it's very impressive, but i know a lot of people that can hear all the recording in his 10$ earphone..., i think most people don't wanna just a speaker that sound correct because they already have that experience many times
ATC it's a very good brand, but nowdays there is many good performance at lower price, if you are a mixer you don't need the best speakers or the best tweeter, just a correct speaker to work with, the rest its pleasure imho. But also its a old brand and many people its very familiar with his timbre and i guess because you are familiar with the sound of ATC is easier to work with.
I hear a sound engineer say they like the Yamaha hs8 because he know how to work with the Yamaha speaker