I don't think HHKB caps can actually get irreversibly shiny.
Yes, they can certainly get shiny, mine did, but it is possible to wash them and they look like new again, unlike ABS.
They can wear out too, but only after very prolonged use, something like 10-15 years I'd say.
However, people sometimes miss the key and scratch the sidewall of the keycap with their nails. Most of the old PBT keycaps have these scratches, and this is probably what we can call "bad condition" in PBT world.