I think you are missing the above point.
You think of bitcoin as something with a value in dollars / euro's / etc. The point that was made is that, íf bitcoin reaches 1 million, it probably means that bitcoin has become an accepted currency, therefor there would not be a need to exchange it anymore, you would use it like you would use cash right now.

If not, I'm sure that it will not reach that high of a value without potential buyers, so you won't have to worry about cashing in, if you would really want to.