The project filename is already displayed in the Project Bar if you open two projects with the same name. So I'm not sure where any ambiguity would come from. Either you have one project open and presumably you know which you opened; or you have two projects open with different names so you can tell them apart; or you have two projects open with the same names but then Construct adds the filenames in the project bar so you can tell them apart.
It makes sense to show the filename in the editor in a few more places anyway just to make things clearer, so I've added it to the tab tooltips and also as a tooltip to the project name in the project bar.
As for the project name vs. filename, the project name is used in all sorts of places for things like the app name or the document title for web exports. Quite apart from the problem of "what's the filename of a project you haven't saved yet?", I'm sure not everyone will want that to match the filename - for example whether you use "My game (variant 1).c3p" or "My game v1.5.c3p", you may well want the app name/document title to just be "My game", i.e. different to the filename. If we made it use the filename, I can imagine someone writing an equally impassioned plea that we add a separate project name so they can control it separately to the filename. Not everyone uses software the same way, and we have to design Construct to work for a wide range of customers and usage styles. It's probably easy to critique software design and say it should work exactly how you want it, but it's not always the case that all the thousands of people who use Construct all want that as well. As with most things, there are subtleties and trade-offs to consider.