His insane power came from his stockpile of souls, which he had to kill all but two of in order to rediscover his individuality. So there's IS a good argument for why he should be less powerful now than he was before, he's lost the vast majority of his power stockpile. And he can't get more or risk losing his individuality again, so he's stuck there.
Furthermore, if we're talking level 0 alucard, the larger he becomes to consume more souls, the bigger target he makes, which means he loses souls more rapidly than he can consume them. And his maximum number of souls would still be the same as the population of the earth, which lucy can wipe out in a few hours!
So again, there's no way for alucard to win. Even if he does survive until Lucy dies of old age, she can have descendents, which is the whole purpose of the diclonii genome, each of which would have its own vectors perfectly capable of continuing to tear alucard to shreds.