I understand your concerns about having a default fallback theme available as a “fallback” option in WordPress.
Having a default theme, especially a WordPress Twenty- series theme, is something you may prefer not to have on your server. It goes against the logic of another diagnostic point we mentioned: you should remove inactive themes, where we explain that inactive themes can be potential entry points for hackers and scripts.
Yes, a fallback theme ensures that your website is still accessible in case of an error.
However, you may not want to see how your website looks with a default theme, and the potential risks associated with using an outdated default theme. In such cases, it may be preferable to see a blank page or an error message that you can address and resolve as quickly as possible.