We don't allow you to delete your current default engine, but we do allow you to delete other engines, because you should be able to get rid of things you don't

We don't allow you to delete your current default engine, but we do allow you to delete other engines, because you should be able to get rid of things you don't want. We used to have a "restore all settings" button that would fix this (among other things), but it disappeared in the preferences UI rewrite and though I've agitated for it to come back, it hasn't.