Money should have a store of value but does not need to be a "perfect store of value"... money can loose value over time, so long as it is a slow process, and it is still a store of value.
Much like a bucket that very slowly leaks is still a store of water, it just isnt a perfect store of water.
But more important to the fact is that this is a **desirable** quality... We want money to slowly loose value because that encourages investment rather than hoarding.