Should we just let any old thing that slips into common usage to become the norm?

Yes.

Why not spell it "definately"? It's very common and everyone understands it.

I don't think that quite meets the threshold yet, since most people who do that would still agree that it's not correct. However, it's close, and I wouldn't be against recording it as an alternative spelling.

It's a bit tangential, but English spelling is awful anyway, it bears hardly any relationship to the pronunciation, and I think it's great if it evolves to be a bit less unintuitive.

I suppose you probably do accept the existence of American spellings, even if you aren't from there? So the only difference between us is time, and how many people use a variant. Everyone is a descriptivist, some people just also think they should force their opinions on others, which is wrong. ;)

Common usage the the norm are literally the same thing.

Prescriptivists act like 'the norm' is some ordained perfection and everything in their own lifetime is an aberration, but that's just temporal exceptionalism. Do you really think you just happened to be born at a time when the people writing style guides pointed at the be all the all of the English language and all advances are just corruption?