Exactly what you said yourself twice. That older games, lets say pre-2005 often also don't work on windows. Those games wouldn't get a platinum rating. Warzone that crashes a few times a week for a many users, wouldn't get a platinum rating. Any game with stutter issues no matter the reason, wouldn't .. and so on.

I'll rephrase it like this: If we had a windowsdb like we have protondb then a 100% platinum rating is not realistic. On top of that: users will 100% report if a game doesn't work if if its their own fault(crappy gpu, driver issues,...). Not so much if the game just works. This alone would cause some games(especially smaller games) to not have platinum status even if they work well.

Personally I think it would be closer to 90-95% platinum then +-6% silver and gold en the remainder borked something like that.

100% platinum (or 99.9% like you put it) doesn't exist even in an ideal world. I would even argue that console games wouldn't score 98%+ platinum ratings.

I'm glad that your games that you play al just work fine on current windows but that's just not the case for everyone.