If the goal of srfi-146 is to be implementable on top of srfi-125, hashmap-comparator is the one that prevents hashmap from being the same thing as srfi-125 hashtables. But in general, I think the introspection of comparators is valuable when you're writing a generic data structures, and the lack of it would drag future libraries to be written on top of srfi-146. I think I had a bunch of occasions I needed to retrieve comparator from a generic map.
For the implementers, it's probably not much a barrier to add hash-table-comparator natively just to support srfi-146 efficiently. How important is it to have both (1) srfi-146 on top of existing srfi and (2) allowing hashmap to be hashtable?