I have a theory that when a Miraculous bearer reaches the end of their career, they return the Miraculous to the Turtle Bearer so he can put it away for safekeeping and/or choose the next bearer.
This would be currently Master Fu, but as he isn't quite old enough to have known the former bearer of the Ladybug Miraculous, it would have given to his predecessor, whoever that was.
And probably not even that one, considering the length of time involved (569 years), assuming Turtle holders have a maximum lifespan of 200 years.
This might also explain why the Moth and Peacock Miraculous were lost, something happened to their bearers before they could retire and return the Miraculouses.
I'm interested in why you say the Turtle Miraculous holder is always the guardian of the Miraculouses. I'd think that would be dangerous, since any new Turtle Miraculous holder wouldn't have the necessary experience. So my personal theory is that the duty falls to the most senior Miraculous holder, who gains custody of the chest.
Regarding the question of retirement, I think that kind of describes what Master Fu has already done. He isn't exactly active in what he does, and states that he's too old to be a hero. That sounds like he has already retired, and is simply keeping the chest as a safeguard. Of course, since he's the guardian of the chest, he doesn't have anybody to return the Miraculous to, but I imagine that Miraculous holders don't return their Miraculouses unless it's necessary. They wouldn't want to have to be separated from their kwami, after all.
The reason is simply because the Turtle holder lives longer then the others, if Master Fu's age is to be considered.
So eventually the current holders will get near the end of their lives and no longer be physically capable of using their powers, at which point their Kwami will advise them to return their Miraculous to the Turtlebearer and most likely say a final goodbye, as they won't be seeing their partner again.
Likewise, I am assuming that the Turtlebearer is never intended to fight, their role is more that of support.
While they can battle enemies, I suspect this is only a matter of last resort, they probably only transform to protect the box from thieves and/or recover stolen Miraculouses.
And it seems like time to learn is never a factor, based on how quickly Marinette and Adrien picked up their abilities.
I assume all you need is a half hour chat with your Kwami explaining how to use your powers and then you are good to go.
The rest is "on the job training", so to speak.
I assume that there isn't a safeguard in that eventuality
Remember, Hawkmoth is the first Miraculous bearer to go evil, before he did, it was simply unknown that Miraculous powers could be corrupted by someone with malevolent goals.
But unofficially, I assume distributing the remaining Miraculous and especially the Ladybug/Cat Miraculous is the safeguard.
If the Turtlebearer abuses their power, the combined force of all the other bearers would defeat them.
It might not even require that many, given that it was stated that the Ladybug and Cat Miraculous are the most powerful, they could defeat the Turtlebearer without help if necessary.
Given that Ladybug has beaten Cat Noir at least twice when he's been forced to fight her indicates if necessary, she could defeat another bearer all by herself.
But if I was doing it, I'd have an protective measure on the box, that it could only be opened with those "pure of heart".
That if your intentions weren't pure, the box would stay locked and generate an energy shield to prevent you from opening it to get the other Miraculouses inside.
They've never said if Miraculous holders can do this though, while powered by magic, I'm uncertain if they can actually cast magic spells.
And on the subject of that box, I wonder if it's gone through various redesigns over time.
The current version uses a basrelief lock, a 4 digit PIN code and a clockwork mechanism to open.
Also, given it is disguised as a gramophone means it can't be more then 139 years old, as this was when the first gramophone was invented.
Given that it doesn't resemble that model means it might not even be that old.
And now I am wondering if Master Fu saw the invention of the gramophone first hand, he'd have been 41 at that time.