Re: "virtually zero" How are Morgan Stanley counting?
Bitcoin may or may not be a viable currency (I'm not vested in it in any way or fashion myself), but I find your argument that Bitcoin is "structurally incapable" as operating as a mainstream currency based upon its scarcity to have a very serious flaw.
If we took the same argument and applied it to a precious metal such as gold, when there isn't enough of it to go around to pay any more than a minuscule fraction of the gross world product, it would mean that gold is also worthless as a currency.
Instead, I think you will find that a great many people value gold for its monetary value and not its physical properties.
Something is only worth something if someone else wants it. That demand imparts value. That goes for your house, your art, your car, your gold, or even your Bitcoin. If it is portable and uniform then it can easily be used as currency - just as cigarettes are used among inmates in prisons.
Bitcoin is portable, uniform in nature, and demand has imbued it with value. For better or worse it is for all intents and purposes a currency, and as long as there is a demand it will continue to be viable as a currency.