In many ways, template parts are considered the equivalent of widget areas. Through template parts, users who have the capability to edit_theme_options will be able to add blocks of content to their sidebars/headers/footers just like they added widgets to their widget areas.

When a theme with widget areas is deactivated, the widgets that the user added will go into the Inactive Widgets section, enabling the user to transfer those widgets to their new theme easily.

When a theme with template parts is deactivated, the user's customised template parts become quite difficult to retrieve. They will not show up in the Site Editor at all, meaning that Appearance > Template Parts is the only way to retrieve them. Even then, they would have to open up two browser tabs to transfer their blocks to their new theme.

Clearly, there will be a need to be able to transfer template parts between themes in a more seamless way, like is possible with widgets. However, when a theme is not active, it is possible to delete that theme.

How can this be solved without risking content suddenly disappearing because of a theme being deleted, and also without denying users the freedom to delete old themes?