This criticism is not meant to say that each BTC will become practically too expensive to buy; we all understand that there are granular subdivisions. It's meant to say that a fixed money supply means that as usage of a currency expands, demand for the currency will increase, and supply cannot match. If the value of BTC continues to rise, you'll never want to buy anything with it -- why not wait 10 minutes for the price to drop? In the same sense, you'd never want to sell anything for bitcoin. You'd need to update the price several times a day! It's not very useful as a currency. This is a fundamental flaw with BTC, because there's absolutely nothing anyone can do to fix it, it's hardcoded.
If we're all being honest with ourselves, people are buying BTC now because they expect the price to go up -- not because they expect to use it as a currency in the future. I don't think we know it's a bubble. No one can really know that. I think we do know that it's being used only for speculation at the moment, which makes it a commodity, not a currency.