one is now very hard-pressed to find a theme developer who is rolling their own options panel after the Customizer was introduced as the new standard

Is this a joke? Perhaps that is true on the WordPress.org repository, since integrating options into the customizer is a requirement for themes that want to be listed there, but the vast majority of themes on other marketplaces still roll their own admin panels.

It's also apples vs oranges. No one needs to disable the customizer for their site's features to work. It's just an additive experience. This is much more similar to the replacement of the media library, which saw a mass die off of popular plugins in its wake.

The issue with Gutenberg vs Classic Editor is that they are two completely different and mutually exclusive development environments. So, either plugin developers stick to the subsets of metabox interfaces that aren't broken by Gutenberg, or they build systems that aren't compatible with Classic Editor, or they do twice the work.

Realistically, that means plugin devs are going to pick a side. Those that pick the Gutenberg side will see a significant reduction of userbase, but will also make it much harder for users of Classic Editor.

What is really crucial from Matt/Automattic is not just a statement that classic editor will be supported indefinitely, but a statement that it will be supported by all Automattic properties, such as WooCommerce. This is especially crucial with WooCommerce's history of implementing breaking changes.