I'm not a linguist by any means, but this is a completely incorrect answer.

In my defence, the comment is intended to encourage improvement to the question, and makes no attempt to answer it.

No, it's actually not entirely subjective.

I never claimed it's entirely subjective. I'll admit that 'very' might be a little strong, but I stand by the claim that there's a significant degree of subjectivity to it.

I'm aware of how dialects spread and develop, but being able to trace dialects back to some point on a family tree isn't the same thing as measuring how much they have diverged since that splitting point. There's certainly going to be a tendency for dialects that split recently to diverge more than those which split longer ago, but it's far from a perfect correlation - there's a significant influence from other factors such as the level of contact between the diverging dialects, and the contact each dialect has with external influences. Take Indian English as an example, which I would (non-expertly, I should stress) argue is more divergent from BrE than either American or Australian English despite not splitting off any earlier, (presumably as a result of many L2 speakers and a lot of contact with distantly-or-not-at-all related languages?).

I don't want to drag this post out too much for fear of giving the impression I have any expertise in sociolinguistics beyond a vague awareness that it exists and feeling that "this is kinda interesting" - I don't, and tried to word my earlier post to avoid giving any impression that I do. But I'm not convinced that the genetic relationships you're talking about really address the question as I interpret it.