IMO, this would be a wasted idea to implement. It wouldn't change anything or serve any real purpose.
Any given number format has a finite upper limit for the number it can represent.
1m is just an arbitrary "neat" limit below the actual upper limit that it can currently support.
Raising the limit would simply see people aim for a new one.
214,748 Gold, 36 Silver and 46 Copper was limited by the use of a 32-bit signed integer, which had a range from its lowest to highest value of 2,147,483,647.
It was stored in copper then, and it still is.
Nobody is asking the most important question- Where would this Platinum come from? Azeroth currently does not have ANY Platinum ore/bars/whatever. Theres copper, silver, and gold, but there is absolutel no Platinum ore.
It doesn't benefit WoW at all, everything would still be the same, people would make it out. Current stuff would still cost the same equivalent amount in platinum.
It would just make a complete waste of time for blizzard to put any effort into making this change, and as someone said before it doesn't go hand-in-hand with lore, as lore has never referred to platinum for a currency.
If you wanted to use "Platinum" then you would implement it before the game is initially released, adding new currency does nothing gameplay wise, at all.