Sure, you can say that the Solar system ends where it stops being gravitationally dominant. But think about this, we say we left Earth for space when we actual

Sure, you can say that the Solar system ends where it stops being gravitationally dominant. But think about this, we say we left Earth for space when we actually leave Earth's atmosphere. When you cross heliopause you are actually leaving "atmosphere" of solar system. Within it it is dominated by the particles from solar wind. And the change is very abrupt and well defined (but only in the direction of the motion of the Solar system, ie. where Voyagers are) in contrast to how Earth's atmosphere just gradually thins out and we have to get into some sort of agreement when it actually ends.