I am not opposed to 4 tbh. In practice, the OSMF tiles are widely used far beyond their initial purpose: in the Greek section of the forum we endeavoured to document some usecases of OSM data in Greek local contexts and by a large margin OSMF tiles dominate.
What this would entail in monetary costs is a valid argument, but once again pretends that the use is not already being made with the current policy anyways. Its not like this new layer will be easier to implement and therefore see a skyrocketing of use. If anything it would put less strain on OSMF resources that third parties would use a less frequently updated tileset, am I mistaken?