Saying you are always a beginner is cheating, using a philosophical point. Of course we all strive for shoshin.
But it's such a common thing to do among instructors and traditional martial artists that in and of itself it becomes a form of bragging. How did you earn the right to be so modest? If modesty is an indication of mastery, aren't you just saying "hey look how advanced I am"? Rather than modesty, it's just dishonesty.
No, I'd argue that in many cases it is an acknowledgment of the complexity and depth of what it is you're trying to learn. Some things are never really mastered. So the expression isn't just about "beginner's mind" but an awareness of the incredible depth of the area of study.
FWIW when I first started polishing I did a few swords and thought "wow, that's not so hard." Then I learned a bit more and looked at what I'd done. Ouch. Then I got more training. Ouch again. Now after a lot of years doing it I realize how much more there is to learn with each and every sword I work on. And I'm not even working on antiques in terms of blade restoration... And while I have people who like my work (which is really nice in many respects) from where I'm sitting I just see how much more I have to figure out. I had a sword polished by a living national treasure for a while in my safe. I studied that over and over again because it literally exponentially expanded my awareness of how very much I have to still improve.
Frankly with this stuff (aikido included) if you think you've got it all figured out you're either the one-in-a-million supremely gifted and magically enlightened "one"
you've simply missed something.
Smart money is on the latter...
It's not false modesty most of the time IMHO. Just relaxing into a realization that this is a *really* long road... it is a matter of an expanded (and greatly lengthened) perspective...