TL;DR The default ability to easily delete a Parent Theme can completely break a site. Shouldn’t be so easy to do.
I was quite shocked that MainWP would happily allow me to delete the parent of my active child theme.
Working in MainWP allows for sweeping changes, and it’s my contention that any parent of the active Child Theme should not be removable. They should be locked by default and, if one really wants to remove it for some unimaginable reason, one should have to affirm multiple times.