These coins would be removed from the market and everyone would be trading in 0.X values.
What's the problem?
Is that a very practical way to go about trading currency?
Not any more or less practical than trading in X values, or in X00 values, or in 0.00X values.
Bitcoin is digital and infinitely divisible.
Once 0.0X transactions become mainstream, people will probably tend to use more practical names like bitcents or whatever. As in "Hey bro, here's that 3.5 bitcents I still owed you" (referring to a transaction of 0.035 BTC).
Really, people should stop worrying about that 21million limit. A limit means nothing if the currency is infinitely divisible.