critique:  Bitcoin is too volatile to be a day-to-day currency, plus it takes too long for transactions to settle, and sometimes has high transaction fees, so it's a failure as money.

rebuttal:  It's true that bitcoin is not a viable global every-day currency today (though it settles $billions equivalent in transactions every day). And it may never be. But that's ok. Gold would make a much worse day-to-day currency, and yet humanity finds over $10 trillion in value in gold's fundamental properties. Furthermore, bitcoin's digital nature and programmability mean future innovations can be built on top of bitcoin's base layer, potentially enabling direct frictionless instant global payments at scale.

It is commonly accepted that a "currency" has to be 3 things:
1) a medium of exchange
2) a unit of account
3) a store of value

Bitcoin critics argue that it directly fails on #1 and #2, and that its trading volatility (critique #3) invalidates #3. Perhaps "currency" is a misnomer for bitcoin today. In any event, bootstraping a commodity like bitcoin from nothing to a global currency that satisfies all 3 conditions would likely require it to go through various distinct phases of development. Nick Szabo examines this process in his 2002 (pre-dating bitcoin) essay "Shelling Out -- The Origins of Money".

Bitcoin is arguably in a phase where it is emerging as a store of value, which can serve as a base on which to build the other two pillars of a good currency. But even if that never happens, digital store of value is a huge market, and an asset doesn't need to be a globally accepted means of payment in order to excel as a high-marketcap store of value.