The problem isn't the existence of the Customizer. I think it's a slick idea for people with little to no CSS knowledge to make tweaks to their site layouts on their own, saving them time, and perhaps even some money because they can make the changes themselves instead of hiring a dev or designer for a few quick changes.

The problem is WP setting things up and eventually forcing the Customizer to the be default admin interface for everything, which if you read the various updates and discussions and commentary is pretty much what they've stated they intend to eventually do, years down the road.

Forcing the Customizer to be the admin interface by default for everything without giving people who are more savvy admins the ability to choose otherwise is the big issue, and I don't see the Customizer becoming advanced enough to be that streamlined and efficient.

This has nothing to do with the diversionary excuse "oh, this is just an educational issue, they'll get over it", which is incorrect, dismissive, and blatantly disrespectful of the people trying to point out the misguided all-or-nothing attitude of "The Customizer Is Mother, The Customizer is Father" that the people making decisions seem to have adopted, needs some fine-tuning before going forward.

It's that dismissive and seemingly intentional deafness towards what the Customizer complaints really are about that has me confused. It's not that difficult to see, and the fact that so many of us who have been pointing out the shortcomings are called out as being luddites or afraid of change and other childish names borders on arrogance. The folks pointing out the shortcomings aren't the ones sticking fingers in ears and yelling LALALALALALA while ignoring the concerns of a lot of smart people.

The Customizer has its uses and functions, but as a complete replacement for the admin interface for everything is IMO definitely not the way to go. But that's where we seem to be getting pushed towards, for no clear reason why other than "gotta compete with Squarespace".

If that's the vision, then make Customizer the default for WordPress.com and let the millions of people with self-installed sites who have legitimate concerns about how Customizer will affect the administrative functionality of their sites have an option to go a different way.

At this point, it may be moot. 3-5 years from now, all backend admin functions will have been shoehorned into the Customizer, there will be a few dozen plugins that will switch the admin interface back to something more usable for those of us who prefer functionality over "drag & drop & make pretty only" content management, and the Customizer arguments will continue until the end of time, while many of the media management issues that have gone unremarked upon for years will still exist.