Sorry, what? You are presuming too much. I can use whatever file names I want for what ever reason I want. There are no constraints in that sense. One isn't always working within the constraints of one project.
Sometimes, when already editing, I want to refer to other projects, for reference, to cut code, to fix a common problem etc, and if the file in question is a .bas file that I want to look at, then I would like to be able to see the available .bas files in the open dialog, you do that by having a *.bas filter available.
Sometimes I want to save a separate version of a bas file, I want to see what bas files are already in the directory from the SaveAs dialog so I can select a suitable name. You do that by having a *.bas filter in the SaveAs dialog.
If it comes to that, I don't always want to open an sbp as though I am opening new project, all I want sometimes is "just for reference", you know "how did I do this last time, 5 years ago". But there is no way to do that either. There are two reasons.
1) there is no close option, or close all; that could be used to infer that a new project will be loaded, and if not used that other opened files are not necessarily part of the project
2) there is no separate project file; this means you have no separation between "mere code" and project definitions, we should have free access to "mere code" so that we can work however we need to.
But all that is needed as a first step is the trivial addition of a couple of filters to dialog boxes. There is no side effect as "other files in the editor" are never referenced by the current project, "other files saved" are never referenced by the current project. I'm not asking anyone to write "War and Peace", LOL.