Plausible deniability is not anonymity. Mixing your own coins with themselves is not really mixing at all. Okay the most secure sort of mixing is with blind signature "tokens". You give a bitcoin to the mix, it gives you a blind signature signed token saying "IOU one bitcoin". Because of the blind signature algorithm, it can verify its signature at a later point in time but it does not know what the signature on your token actually looks like. This offers perfect unlinkability, but only to within the set size. If you use such a mix and only you use it, the mix operator (and anyone watching) will know that it is you redeeming the signed IOU for a bitcoin to another account, because you are the only one who ever got an IOU. Thus, even with provably unlinkable mixing systems, the anonymity provided is exactly correlated with the size of the user base. If a thousand people got tokens from the mix, someone redeeming the token for a bitcoin to another account could be any of the thousand people who got a token. Of course you could argue, but it was not me who redeemed the token it was someone I traded it to for one bitcoin worth of goods!!! But that is more plausible deniability than anonymity. If a thousand people had used the mix you could say "But you have absolutely no way of proving it was me who redeemed this token, because there were a thousand issued and only the people who received them know what the signature on theirs looked like!!"