I don't doubt that some accordionists started on piano accordion and didn't play piano first, but I expect them to be in a minority. It's also quite likely that Phil Cunningham started on piano accordion instead of button accordion mainly because there would have been far more choice of instruments and teachers available for that instrument.

I was talking about why the piano accordion was designed that way in the first place, and why it's so popular. The button accordion layout is actually far more logical, and if you already know how to play it, the piano keyboard offers little if any improvement.

One advantage of the button keyboard layout is that it packs more octaves of notes into a limited available keyboard length: the piano keyboard is fine on a piano because the keyboard can be made much longer.