How sure are you that BTC will succeed?
Let me elaborate.
Succeed as in it will reach $5,000 or greater and it will reach main stream adoption.
As for figures, let us just say at least over 20 Million users from the current estimates 2-3 million users as of March 2014.
As for time frame, lets say within the next 2-3 years from this post's date.
Phrased like that, in its generality, I wouldn't know how to answer your question...
"Mainstream adoption" as a daily used currency when you do your groceries?
"Mainstream adoption" for Internet-related business transactions?
"Mainstream adoption" for Internet-related business transactions, but not for the end user but by being the backbone of the transfer system (similarly to how Linux effectively is the OS that runs the Web, even though the majority of users has another OS)
"Mainstream adoption" as a store of value (assuming volatility reduces, or becomes more predictable), but not as a daily used currency?
My point is, mainstream adoption can mean a huge number of things, and the fact that I believe one or the other scenario is possible or even likely has no bearing on how likely I consider another mainstream success scenario.
EDIT: if you mean however how likely it is that there will be 10 times as many users as there are now, where "user" allows for our current usage (speculative asset, some trading/payment activity), then I'd say 90% or more likelihood.