Any of the morons who reported "What happened to my layers?" through support channels.
The typical use case for most people using GIMP is on a file that doesn't have any layers in the first place. If that's the real reason, then they could produce the warning message only when the file contains layers rather than all the time. Perhaps a second warning message, that can be turned off, if the file format is lossy.
If the names of the "Export" items on the File menu were changed to "Flatten and Save", would that satisfy you?
Just putting the word "save" on the "Export" menu item would still leave you getting warnings about not having saved it. You'd have to put the word "save" on the "Export" item and have it actually count as a save for the purpose of deciding whether you need to get a save file warning.