First, I'd like to say that some Reformed Baptists will conclude that baptism replaces circumcision, and yet they remain credobaptist. For example, see David Kingdon's The Children of Abraham amd Paul K. Jewett's Infant Baptistsm and the Covenant of Grace.

Secondly, Will, I would not use the argument that, since we cannot infallibly baptize disciples alone, that that negates the purpose. It's like saying that we should not fence the Lord's Table at all because when we say it's for believers only, there may be nonbelievers partaking of it.

True godliness is a sincere feeling which loves God as Father as much as it fears and reverences Him as Lord, embraces His righteousness, and dreads offending Him worse than death~ Calvin