I know that there are no plans now to remove support for the customizer but over the long haul (and I'm talking 5 years or more) I expect it to vanish as block themes take over.

And I suspect there will be away - if not already - to convert a classic theme into one where the customizer is not used.

For those who have said, "The customizer must die" your wish will become reality. It was inevitable since Gutenberg 0.1.