I completely agree with this. There is little reason to have Uncategorized behave special. Sure you always need a category to be assigned to a Topic, but if someone removes all categories from their instance, surely an error will occur when they try to create a new topic and can’t assign a category.
By default, Discourse should continue to ship with an Uncategorized category, just make it a regular one though and not special. That way, anyone could technically delete it and just create another category that would be their default or whatever.
I’ve seen a fair share of uncategory logic when I’ve been in and out and I must say, it always felt like walking on pins and needles when you had to alter code that was dealing with Uncategorized.