I do believe it can be used as a medium of exchange right now. However, yes, it is very limited due to current price instability and lack of global scaling. The jury is still out on whether that'll change.
I do believe != it is
Fair enough on the phrasing, but when I say "I believe", I mean to say that I reason that isthe truth but like to try to remain modest and not throw around very bold claims.
Anyways, solely by the fact that some people use it as a means of an exchange - Bitcoin is a viable medium of exchange.
Is it perfect? Is it great (considering current legal realities etc.)? Can it be used by everyone on the globe? Currently the answer to all three is "no", but it has a non-negligible chance to improve in all these aspects.
It's proven it can do all of these things except that it can be used as a "medium of exchange". It doesn't mean it's not possible, it just hasn't proven it yet. All crypto has proven is that we can create a digital version of bullion, that doesn't have all of the same issues as trying to barter bullion.