It sounds like your primary criteria for the success of Bitcoin is that significantly more people use it than currently do. Is that correct? Would you consider Bitcoin successful if, say, it kept the same number of users as now (give or take a factor of two)?
My primary criteria for the success of Bitcoin is not just that significantly more people use it then currently do, however I do believe that this is an important criterion, but quite pointless by itself if Bitcoin became like Visa for instance. So increasing adoption while maximizing decentralization and financial freedom would be a more accurate criterion for me.
Perhaps it's good to separate between necessary conditions vs. sufficient conditions for success. You seem to be saying that significantly more people using Bitcoin is not sufficient, because decentralization and financial freedom are necessary.
Propositionally we could assert that the following would be true in every world:
SD: "Bitcoin is successful." -> "Bitcoin is decentralized."
SFF: "Bitcoin is successful." -> "Bitcoin supports financial freedom."
Or, if we want to be classical and avoid implication, the following are both always true:
SD: "Bitcoin is
unsuccessful." or "Bitcoin is decentralized."
SFF: "Bitcoin is
unsuccessful." or "Bitcoin supports financial freedom."
I agree that both decentralization and supporting financial freedom are necessary. (In fact, now that I've clearly stated this, I'm not sure I currently consider Bitcoin "successful." I would tend to consider Bitcoin to be successful at the moment, but I also believe that mining is currently too centralized.)
Is a signficant increase in adoption necessary? By "significant" here I mean something more than twice the current users. I haven't thought deeply about the specific numbers. I can try an absolute percentage just to be precise. Is it possible for Bitcoin to be successful if less than 0.1% of the people in the world use it? Actually, I'm inclined to distinguish between "use it directly" (meaning the individual holds the private keys and sends transactions to be included into the block chain without using a third party service) and "use it indirectly" (via third party services). "Mass adoption" would very likely give different percentages for direct use and indirect use (as has the current level of adoption). You can decide if "use it" means "use it directly" or "use it indirectly" -- and you can adjust the percentage -- before deciding on an answer.
(Incidentally, while I find the discussion interesting, I can't be online often. If I don't reply quickly, or even daily, it's not with ill intent.)