The ‘commonly accepted’ argument is not a great argument, and if it’s your main or only or pillar-argument against bitcoin then you will likely become a bitcoin supporter soon (unless you’re a SD sock).
What does it really mean to be ‘commonly accepted’? It just means that many people will accept X as a medium of exchange because they know that they can turn around and exchange X for what they want.
Let me flip the scenario now. Suppose we go back a few hundred years to when the Cowrie shell was being used by primitive cultures as a form of money, and along come the Spanish with a new technology: gold money.
According to you, gold cannot be money in this scenario, it’s not commonly accepted in this culture. But time passes, the Spanish explain the wonders of gold, and do business in gold with the people, and gold-use as money is on the rise. It ends up outcompeting the cowrie shell because of its superior properties in its use as money compared to cowrie.
Soon demand for gold exceeds demand for cowrie and better money outcompetes the worser.
Now, compare bitcoin and fiat. Many people prefer bitcoin for a whole host of reasons to fiat, reasons ranging from privacy, coolness-factor, investment hoarding and the expectation of deflation, and the more ethereal removal of 3rd party trust and thereby avoiding the whims of bankrupt politicians (aka Cyprus).
The market will decide if bitcoin is a better money than fiat, over time. And then comes a tipping point, as it surely came for gold among the cowrie-using society. The tipping point is generally said to be the 15% - 18% adoption rate within a population. This doesn’t need to be judged on a worldwide basis, can be culture by culture, even city by city.
Bitcoin is certainly not there by any means, but you cannot sustain the argument that because it’s not there right now that it can never get there. If that were true nothing would ever achieve a tipping point. But if it achieves that someday, then we can say surely that it is now commonly accepted, because at that point it’s likely that any person would accept bitcoin because of reasonable surety that they can exchange it for anything else they want.