I think the frame tool in principle already does the same job as e.g. the Krita canvas. Even in krita, layers are sized dynamically - nothing prevents you drawing outside of the canvas, and the result is still there, out of view. I guess the issue is discoverability and expectations - in most pixel editing software, the canvas is a central and visible thing, whereas in MyPaint it’s an option not enabled by default. Users might not associate the term “frame” with what they are looking for.
I agree, the Frame tool seems pretty much exactly what one would need to have a regular canvas. I rarely use the frame tool and almost never have the “options” panel open. So, embarassingly, I never realized there were settings for DPI, inches, etc when defining the frame:
Touchscreen support would be great though. I know someone has a fork with some basic support in it, but there are a lot of issues to grapple with still.