I feel blessed that at my old age I am still fast. Seriously.
This is true, but if you use a metal cover the body has to be of metal too. If not you are just shifting the breakage point to the body of the key (or better to the junction between body and cap.
That may be a better idea, however this design requires a good material for the axis of the retractable / shiftable device. There was once a key that came with tails (promotional) that used the system of an axis on the key that moves a hull that covers the key from the side. Drawback is here that with the time, also here the force to open it to the side is getting less.
Now if you use a connection of a traditional cap, e.g. a rope, that has to be changeable because whatever material may wear out. I would look at textile (nylon) solution or metal (sphere chain), as rubber or plastic will soon get unstable or will get micro-fissures, then rip off.
Any mechanical lock (as said before) might transfer the problem to the body of the key.
As you see, it is not so easy. But I am quite positive to “retractable” or “side move cap” as of the aforementioned design of the tails key.
Or, i could imagine a metal frame with a “buttonhole” as to say, that can hold the bullet chain / nylon for the cap. The advantage would be that the frame would join to the looks, it would hold safely the both usb outer shells without leading to a bigger body. The key as a whole would be more resistant to stain (albeit also a bit heavier).
Every design has as far as I see advantage and…disadvantages.