But there are many many other themes in the same boat. It is likely that there will be legacy support for many years to come since there are so many existing sites using the current theme technology. There are not many left supporting only the legacy interface (which means they support only the 5 or 6 standard WP customizer options), but they still run. No doubt Customizer based themes will continue to work for a long time. But all the emphasis of the WP crew will be to support the new block type themes.
While the current core WP development seem to have little respect for existing themes, as well as little respect for the “amateur” web site builder, and are really only interested in the new things (which they do believe will make WP better), they do understand that they support a huge percentage of all websites in the world, so will keep supporting the legacy codebase.