These are the two most common arguments against pure Flying-types I see, and they're both BS.
1) The "regular bird" argument makes no sense. It's a double standard. Goldeen is just a regular fish, why isn't it Normal/Water? Caterpie is just a regular bug, why isn't it Normal/Bug?
2) The Roost argument fails to realize that they were making birds Normal/Flying long before Roost was invented. So that's not the reason.
This right here, man. There's really no reason for those normal/flying types to not be pure flying.
While we're at it, "it was originally supposed to be the Bird type" doesn't answer it- why wouldn't the "Bird" type be used singly instead of paired every time? And if you mean they were originally bird/flying types and the bird type got switched out for normal, well, that doesn't answer it either- they could just have been changed to pure flying. Even if they weren't the birds in later generations who were made without the bird type in mind could have been pure flying.
So basically there is no reason whatsoever that there can be no mundane pure flying types, aside from Gamefreak being weird.
Because there are like a billion different types of irl bugs and sea creatures. There's enough diversity that they don't need to do the normal/[type] thing. How many irl animals can you think of that can fly and aren't birds? Not too many. Normal/Flying = Bird or Bird-like, Plain Flying = Everything else.
Um, no, there's plenty of varieties of birds out there, and Pokemon already has quite a few birds who aren't normal/flying. And even if birds were completely devoid of variety both in the game and in real life, why does that consign them to always having a secondary type? The two have nothing to do with each other.
If anything, the diversity of bugs, sea creatures, etc means
they should be more prone to getting a second type, so we don't end up with a buttload of pure bug/water/etc types.