Do you not feel that this thread has drifted into the territory of weightless metaphor and pedantry?
Well the way I look at it, when you're a black belt you're a "serious" beginner. From my experience, most students are going to drop out before reaching black belt. There are different levels of being a beginner, you can be a beginner who is just starting out or you can be a more advanced beginner. Being a black belt would be like being a more advanced beginner so all the ranks before it would be earlier stages of being a beginner but the fact of the matter is that first degree black belt is a rather low rank when you look at all the dan ranks and in Japan the rank of first degree black belt is called Shodan which is literally translated as "low man," so there you have it.
The fact of the matter is that in the martial arts no matter what rank you've got or even if you want to throw rank out the window and just look at knowledge and skill, no matter how much you know and how skilled you are you can always learn more and you can always get better. So even the really high dan ranks can be seen as "beginner" ranks depending on your point of view.
But is that a useful definition of "beginner"? Yeah, there's always more to learn, but that's true of every human endeavour. We don't use that qualifier with other fields. We don't look at a university professor and say "they're a beginner," despite the fact that there's more to learn in their field of inquiry.
That saying is an old chestnut in martial arts circles. And I'm not a big fan of it. I'm not saying this is true in your case, but it feels like false humility in many cases. The sort that's really just a veiled way of saying "look what a good, humble martial artist I am."
Even on the low end of black belt timelines, say three years, that's enough time to gain some genuine skill and understanding. There are absolutely schools that issue black belts to people who lack both, but then the black belt isn't likely to be the beginning to anything meaningful for such people anyway. So let's assume we exclude that case.
Having said that, I get what you're saying. Getting the black belt is a milestone. And you're either going to recommit yourself to learning or you're going to say to yourself "no more worlds to conquer" and quit. I've seen both happen many times.
Personally, the metrics by which black belts are issued and maintained are so variable as to be very unreliable gauges of anything at all. But a black belt has either 1) attained some good experience or 2) convinced themselves that they have. Either way, they're unlikely to approach their training with a genuine "beginner's mindset." And that's fine. The former needn't do so, and the latter won't have the awareness to do so.
It's a symbol. And, like any other symbol, it can be honoured, co-opted, misinterpreted, etc. People know what they know. Better to look directly at that than to lean too heavily on tired metaphor.