I agree with Andrea above, here statement, just like this other one from the telemetry discussion, can be made for just about every core addition / feature for years.

This is especially bad with the customizer - which I don't hate - it's just half baked, and ux clunky at the moment more than anything.

However the decision to steam roll previous wp setups, removing functionality from themes and dashboards to force this change - is another example of the core devs forcing their future thoughts on millions - like microsoft stuffing telemetry and ads in windows - wordpress core.

Fact that pushing this in core also removed previously functioning important things - is pathetic - and no explanation either - cost me 4 hours trying to find where I had added custom css - only to find that a wp update had hidden the screen where that custom css was previously added and working - this was just one site. - it took a plugin to re-open the settings that were removed by something that should of been an optional plugin (the customizer)

I see a lot of potential with it though - there are many things to like and love - for devs, editors, designers, and end users -

There are many times it sucks to use though - when you have many tabs open and have to open customizer to do something - and the page is heavy - like with gifs or video or something - and it freezes the whole browser.. and when you are not on a 30 inch screen, and have a browser with toolbars - and can't see half your stuff..

and the things you need to adjust are 5 levels deep.. and you have to backout..

worse thing is the X to close at the top when you scroll down, or are a few levels deep - and you want to get back - intuitively you go to top of screen - and there is no arrow - only an x - so you click that to get back and it just closes the whole dang thing.. the " arrow on the left should be fixed on screen, not hidden when you have a long list that scrolls it off.

adding more pop up color pickers would likely make it better for most, and a hover click on screen that would pull rules into the custom css box would make it easier for average people to change h2's and such.

another cool thing that could of been a plugin -

it's not used on the 20+ sites I maintain - it's actually a hinderance 99% causing extra clicks and costing more time - 1% of the time I enjoy seeing a heading tag change color when I add custom css. however the things I play with need a new window open, it's usually something specific on a sub page, faster to just have that open, and the inspector in another window - and refresh - as opposed to clicking around the clunky - and smaller instant preview.

I do like it for a few specific projects though - so I'm glad it's been created - it could just be removed from 99% of our projects and we would never miss it - and we'd get a few hours of time back in our lives.