Every client requiring something is absolutely no reason for Gmail to require something. In fact, "everyone else doing it" is a bad reason, especially considering some of Google's history.

That's not at all the same thing. That's creating a new message with its own information, history, and data associated with it. What you've described is a workaround to an existing problem that has no reason to be difficult to solve.

Every email client requires you to delete the entire message.

But that doesn't actually remove the attachments as stored in Gmail, right? Just what you see in Thunderbird?

Is it possible to only delete the file, and not the email text?

Now if only there was any easy way to delete those large attachments. Instead I have to use a third party email program via imap. Apple mail lets you sort by size and delete attachments. Unfortunately with gmail this actually makes a second copy of the message, leaving the original and the attachment. You then must go…