In reply to Trent Lapinski (@trentlapinski).

There is a reason less then 25% of themes adopted the customizer in 3 years.

That reason, primarily, is because Theme developers chose not to implement the Customizer. Your number is also inaccurate. About half of all directory-hosted Themes provide a Theme Settings page. About 25% of directory-hosted Themes use the Customizer. Those two numbers certainly overlap, but at a minimum, 1/3 of all Themes that provide options expose them via the Customizer.

There’s absolutely no evidence or number that supports end users preferring customizer themes.

And that's a straw man, anyway. Nobody has argued that end users "prefer" the Customizer to custom settings pages, and that argument was not part of the basis for the TRT decision.

As the former owner of one of the largest theme shops on .org, we sold both non-customizer and customizer themes and the non-customizer themes outsold the customizer themes by nearly a factor of 3. Customizer themes do not sell as well as custom theme options and we sold singicantly less customizer themes by comparison.

Correlation does not prove causation. I doubt that the only difference between the Themes in question was the use of the Customizer API vs. the Settings API.