I always imagined the following was going to happen:

Recent developments allow us to port/migrate widgets, old navigation menus and PHP templates to their block counterparts…..

So… Why not just “update” these old themes, upgrade them to become block themes… With ab automatic migration mechanism where it is needed…

As a result, all the themes would be updated in the sense of keeping up with technologies. All themes would retain their functionalities and design while being block themes.

By the way, this is the only mission/strategy that really is going to prove that block themes are worth for developers and users, when All designs and use-cases can be recreated.
I still doubt if we can do sidebar-navigations as seen in old WordPress themes.