Remember if bitcoin becomes too expensive to use (eg. a satoshi is worth more than a cent) there can always be alternatives like litecoin for the cheaper transactions and bitcoin to store the wealth.
There's no "too expensive" term.
People will buy what they can.
Much, much probably, 21 millionth Bitcoin adopter won't be able to buy an entire Bitcoin anyway.
Yes. Perhaps "buying a bitcoin" will sound like "buying a Billion $" sounds to us now...