As a software developer, I see parallels within Integrated Development Environments. Gimp exporting to a lossly format is akin to building software. Gimp saving is akin to saving the project and source files back to disk. If my IDE ever let me save the compiled binary without updating the project/makefile(s)/sources or a big warning that it was going to do so, that would be bad.
So I see this the same situation applied to Gimp.For people doing minor edits or simple rescaling (my use case), maybe it makes things a little more complicated. For someone who has put a week of work into a video game box cover, that would be terrible.One idea would be similar to IDEs - the XCF is the source, and JPEG/PNG/thumbnails are the output. The export button effectively becomes 'build' then and generates everything you've listed.