Nope, it's not an issue at all; it's just for user-convenience. Whether convenient or not is of course personal but some modern editors like notepad++, ultraedit ... already doing this for some time. VIM does not detect that either, but gvim did give us a warning about file no longer there. Here I'm just trying to match some modern editor behavior. One sample issue is that when switching between git branches I sometimes find I'm still editing a file that's already not there (moved/deleted/included-into-other-file by others), till I do "git status" or "git add" and then find out the fact.
Of course it won't be difficult to add an extra mode to do that but I'm just thinking it should be easier to fix it from the origin. Remove that test condition and modify `ask-user-about-supersession-threat' to detect file deletion.
Well, if no one think it's a good idea I will draw back my proposal then and patch it in my own local system, or maybe a minor mode then. Thanks!