Yes, we can't be sure, any more than you can't be sure that a meteor made of solid platinum is not about to h it your house...but in the real world, it doesn't happen. Sure, someone could create their own aikido organization where you only get a fifth kyu after six or seven years of dedicated training. Think anyone's done that? Or do you think instead that practitioners of other martial arts are uniformly weak and worthless?
It's not just an aikido thing, but it is arrogant and foolish to dismiss the effectiveness other martial arts of which one has no more than a superficial knowledge.
To protect the innocent, I don't name names.
In my individual case, having slightly slower reaction times than "the norm" (I've actually been tested in lab situations while in post-grad physical ed studies), I'm fairly sure that I'd be struggling in an encounter. That's me.
When (on the rare occasion) I visit other MA, I'm not there to fight or challenge anyone - I'm far too old for that and - I'm there to see what they're about, and to see if I'd be interested in learning their "stuff" - so I don't test them on their effectiveness against my Aikido or test my Aikido against their "stuff" - I try to learn some of their "stuff"...
The sensei I was describing earlier has had exposure to many different things.
Pure platinum meteorite? That would be neat! I'd better not be home when it hits.