That “overbearing assumption” is made by practically every other X-media editing app, for good reason — you can save your work and continue/revert stuff later. If all you’ve ever used is MS paint then I guess it’s an unexpected thing, but any other image/video editing app, digital audio workstation, 3D modeling software, etc… work exactly the same way. It would be unexpected to me if “save as” exported a flat file.

The difference between exporting and saving is extremely important, and is something the user must understand in order to prevent data loss.