I don't believe that they meant 'scarcity' in the way u are interpreting them.
'Lacking', meaning none at all! What we are addressing in this country, is 'arbitrary departure' and that is the difference.
How many other things can we approach in the same manner? Psalms vs hymns?
With respect to your first response, I don't think that you are understanding my point. Scarcity, as used by Bavinck
et al. certainly means the inability to procure wine and bread, such as during famine, in which case other substances may be substituted as elements without guilt. Clearly that is not the case in American churches today. However, my suggestion is that
as a lay communicant, who is unable to procure the elements for oneself, you are placed in the same position as those churches in times of famine. You are wandering in the wilderness in a church that is depriving you of wine or, less likely, bread. Thus
you, as the communicant, may partake of the irregular element so long as your conscience allows. This does not relieve those who, in shepherding the congregation, arbitrarily depart, as you put it, from the prescribed elements. That this is not what the older divines meant I grant, but it seems a reasonable extension of it.
That is, of course, if we grant the older position. Perhaps you do not, for if you view grape juice as a violation of the RPW then surely it is not authorized even in times of famine where bread and/or wine cannot be procured. When Israel was separated from the temple in the divided kingdom, they were not authorized to set up their own high places for worship. If the use of other substances nullifies the sacrament itself, then it is better simply to not have the Supper until such a time as the Lord relieves the famine. To use your own example (and to demonstrate how different the cases are), if we find ourselves bereft of Psalters it doesn't authorize us to sing hymns.