advocates who defend criminals are generally pleading for abolishment of the death penalty, not for abolishment of the life long prison sentence.

Convicted criminals still have certain rights enumerated in the Constitution. Popular demand should never change that-this is a case where majority sentiment is inappropriate because it is by definition biased.

Your argument could be easily disproved by offering death-row convicts the option to have their sentence commuted to life-I would submit that most (although possibly not all) would take the offer.

Also, judges and juries are human and make mistakes. If that mistake is later uncovered, a living prisoner is helped by that-a dead one is not.