The most important and essential quality of 'successful currency' is

"The essential quality of successful money, through time, was not who issued it—or even how portable or durable it was—but rather the number of people willing to use it. "

(Quoted from Digital Gold).

I wanted to post this quote out as I think it focuses on a key aspect - total users (and thus value to those users) really, ultimately will determine the success of Ada as a currency at least.
Whether those ‘users’ are a bevy of dApps with smart contracts and ICO’s built on Cardano platform, or straight out currency use across the world, or both, I think it’s correct that success will be driven proportionate to total users.

Note that in an interview I watched today with Charles, he mentioned that it’s possible that they could create a value stable currency on top of Cardano for everyday use.
I think this would resolve the dliemma that Ada holders want the value to go up, but if you are holding a currency that keeps going up, you don’t want to use it to purchase things (and end up with something like BTC $100Million pizzas…).
Thus, Ada would be like shares in the Cardano platform, and ‘stable currency’ would be the daily use case with merchants.
Might be an ingenious resolution.