Can these be saved up and used to purchase future (purchasable) expansions? Or are the Platinum Coins reserved strictly for cosmetic items and battle passes?

I bought the Deluxe edition and it is allowing me to use 1000 Platinum to upgrade to the Ultimate edition, so wondering if same would apply for expansions. Anyone with similar experience with game shop currency and other Blizzard games?

Plats are strictly for battle pass and cosmetics.
The expansions will cost real dollars as expected.

Be aware the "Activate" button for the battle pass does not trigger any warning, meaning if you wish to keep your free battle pass for another season be careful navigating the seasonal menu because if you click by accident you will activate the battle pass without warning.

Thanks, good to know Platinums are not worth keeping or purchasing then.