Put another way, Ockham’s razor says that if we posit a universe where “A” *doesn’t * exist, and we can’t find any way to distinguish between this universe and that one, then for all intents and purposes this *is *a universe where “A” does not exist.
What makes it a “trump” card in many debates is that it’s shorthand for this. Simply, it forces the opponent to concede that they “A” is unnecessary to explain reality.
That doesn’t mean that “A” doesn’t exist. God, leprechauns, the Greys. These are all things that *could *exist and *could *explain all sorts of shit. You can’t argue against what “could” be, and because you can’t argue against it it is a futile tool for determining the truth.
What Ockham’s Razor does is eliminate the “could” and focus on the evidence. If an explanation invokes something that *could *be, but that isn’t *required *for the universe to appear as it is, then that explanation is futile. It explains nothing novel, while at the same time requiring belief in something that is, by definition, without evidence.
Of course lots of things that we now accept to be true once required belief an were without evidence. But that doesn’t make it rational to believe in such things before we have evidence.
That’s why Ockham’s razor is accepted. It’s a shorthand statement of a logical axiom even if not an application of logic in it’s own right.
People are free to reject the axiom, but in doing so they are forced to state outright that their beliefs are not based on evidence. That is pretty much a trump card on many debates.