There seems to be a lot of confusion about default dashboards and autogenerated dashboards.
When you install HA for the first time, you get a global default dashboard called Overview that is autogenerated. Once you take control by editing it, it is no longer autogenerated, but it is still the default dashboard.
You can create additional dashboards, you can delete additional dashboards, but you cannot delete the global default dashboard called Overview. Otherwise new users won’t have a home page for Home Assistant.
Any dashboard can be made an auto-generated dashboard. If you accidentally took control of a dashboard and want it to be autogenerated again, go to raw mode (in the menu in the upper right corner), delete all contents, and save. If you want a self made global dashboard, and have an autogenerated one on the side, create an extra dashboard.
Any user can set one of the extra dashboards to be their default in the personal settings. That will hide the global default Overview dashboard from them. It is a per-user, per browser setting, so that is why you cannot delete the global default dashboard called Overview.
The default dashboard setting is not very useful if you want to change the default for everyone, or only want one dashboard. If your favorite self made dashboard was not created in Overview, the best way to change that is to edit your favorite dashboard, go to raw mode in the menu in the upper right corner, copy al the code and past it to the raw editor of the “real” default dashboard called Overview. Then you can delete the extra copy.
If you want to restore the global default dashboard to an autogenerated one, clear the contents in raw mode (if you want to save what you built there, copy the raw contents to a new dashboard first).
EDIT: newer Home Assistant versions have an option to create an autogenerated dashboard from the menu to create new dashboards.