"They are wrong because" [they pronounced it wrong from day 1].
"The previous examples to the contrary do not count because" [they pronounced it wrong from day 1].
You do realize that circular reasoning is a logical fallacy right?
I understand your point that x number of people doing something incorrectly doesn't make it right, but that doesn't apply in this
specific case because language is a living entity that evolves along with the people who speak it.
Forget how acronyms are pronounced there actually isn't a "proper" way to pronounce anything. There are only ways that are understood, and ways that are not understood. English doesn't have any hard rules because it isn't really one language, none of the natural languages are. English was influenced by its neighbors e.g. Latin, French, German, and those neighboring languages were influenced by
their neighbors and so on. The origin of any natural language is as unclear as the origins of humans themselves.
I know why you don't want to sincerely provide a grammar rule to back up your position of "correctness", because even you would realize that as soon as you clearly state a rule, there would be exceptions to it. But even if you
could provide a rule without exceptions, it ultimately wouldn't matter because language is
descriptive not
prescriptive. A perfect example is the French Academy. Everything in regards to the French language is dictated in writ by the Academy, as they proclaim themselves to be
the authority on the language. And yet even the French in France don't always abide by the Academy let alone the millions of French speakers outside of France. If the Academy thinks it should be one way but the people actually prefer another way, who's wrong?
A language lives in its people. For every person who says "GIF", that is one vote for that way of pronouncing it, and likewise for every instance of the "JIF" pronunciation. "JIF" is certainly less popular, but to say that it is "wrong" demonstrates a lack of insight into how human languages actually work.