How is this observed?
You're shifting the goalposts. This isn't about me establishing a common ancestor; it's about you establishing that there are hard limits to the change of species.
Now, I think your "observable limits" are false, since change that transcends those limits has been observed, however, let's just set this aside for a moment. Frankly, I don't see how the sorts of observations you describe would establish the sorts of limits you suggest, even if we assume the premises of your argument.
As an analogy, say some anthropologist has been observing me for my whole life. He or she would have noted that I've never once been to Europe. I've never even been within a few thousand kilometers of Europe. Now... does this imply that the anthropologist can validly conclude that I'm incapable of getting on a plane and flying to Europe?
It seems to me that your argument employs this sort of reasoning.