well the empire crumbled so there is nothing backing up spetims, and it has been 200 years so no new ones would be minted, so they wouldn't be common enough
Ummm... there's the fact that they're made of gold. As long as their gold content is trusted, they don't need anything to back them up. Presumably Mede's empire resumed coining; if there is a new coinage, then all Septims will have mysteriously vanished, for purposes of designer convenience. Or maybe they'll ignore the issue altogether.
I mean, the gold coins you found in ancient dungeons that had been unexplored since before the Septim empire were still Septims, if you ever found any out in the open (Dwemer coins excepted). And the game didn't bother to distinguish, at any rate. It's historically impossible, but no one really cares.
It would actually be interesting if they had several different types of coin from different eras, with different size, purity, value, design, etc., so you could get your virtual archaeological numismatist on. It would be one way to signify the relative age of the dungeons you explore -- to know, "this stuff must be from the reign of King Dude, since most of the Dudes in circulation were melted down in the chaos following the end of the Dude dynasty."