Stephen,I think you have missed the point Rich. Only God knows who are the elect are. Reformed Baptists baptise professing believers in a similar way that only Reformed Paedobaptists baptise children of professing believers.
I'm not trying to be pugilistic but I understand your point. If the reason for baptizing professing believers was restricted to God's command then (as I already noted) we might simply focus the argument upon that point. Yet Baptists claim that the nature of the New Covenant being made with the elect alone (unlike other dispensations) means that the sign should be applied to the elect and then make the further argument that the reason God commands the baptism of professing believers alone is because of the nature of the NC. That is assumed in Baptist argumentation but never established. No Baptist can logically move from the requirement to baptize the elect to the reason to baptize professing believers.