I think that the better question is about Who are the "worthy recipients" of baptism, instead of the matter of how we define it . Your point made sense to me and I cannot understand why it doesn't to others.
A wedding ring is a sign that a vow has taken place between a man and a woman who have become one flesh in God's sight. This sign could be worn by an unmarried woman or man in order to give an appearance that they are married, even if they are not. It could also be given to a man by his homosexual lover at a "civil ceremony" yet, in both these instances, the sign means nothing at all about the person's state of being truly married. There is nothing wrong with the sign, but the ones who are wearing it are "unworthy recipients" and it communicates a lie about their state. To the worthy recipients, the sign of the wedding ring given as a token of their commitment shows their true condition, that they are truly married. To Abraham whose heart was circumcized, circumcision was a seal of righteousness. To Ishmael whose heart was uncircumcised, it was not a seal of righteousness to him. If a person was not circumcised at heart, their circumcision meant nothing. Likewise baptism given to someone who is a false professor means nothing like the instance of Simon the sorceror.