I think it's a pedantic point, as I believe that language is as language does.... however, there's no reason why it couldn't be like "gold", which is also highly divisible. You need the "s" plural for things that can't be divided (like dollar bills). Thus, we don't get paid in "golds".

That said, semantically, coins aren't usually thought of as divisible things in a modern context-- they are pluralized, so bitcoin will probably be pluralized in the same way.