whereas I also think that 600$ per coin is also far from optima, it is bitcoins biggest unit of account, and it is no problem to use a smaller unit like satoshi or bits or whatever.
There's no reason you can't do the opposite thing and use larger units to denominate values.
I understand what you are saying, but it seems billion count coins don't fair very well these days.
Maybe that's just because there have been no innovative coins which use billions of coins? I don't think having billions of coins really has anything to do with why a coin will fail.