When Sweeney and Anthony first arrive in London (Sweeney after 15 years of being transported) Sweeney sings the same line.
'There was a barber and his wife,
And she was beautiful.
A foolish barber and his wife.
She was his reason and his life,
And she was beautiful,
And she was virtuous,
And he was... naive.
Then he goes on to talk about the Judge and how he coveted his wife, we find that Sweeney was sent away so that the judge could have his wicked way with Lucy.
As he repeats this line in the final scene, I think that he realised whilst serving his time that he should have been more aware of the Judge and his attentions to his wife...he was naive. Then at the end, the audience has seen that he has been so blinded by his thirst for revenge, he has completely missed the fact that his own wife is right there where he left her. Perhaps if he had not been so determined to kill the judge, he might have made a proper search for Lucy, although Mrs Lovett hasn't helped the matter. Her greed and jealousy helps mask the fact that she is still alive.
Maybe if Sweeney had not met her first, he would have found Lucy and there would have been a completely different story!
Well that's my opinion anyway.