“Sorry, but herring is not part of our “Mesorah” simply because it was a poor man’s “fish” in parts of the alte heim.”
It goes back much further than that.
זָכַ֨רְנוּ֙ אֶת־הַדָּגָ֔ה אֲשֶׁר־נֹאכַ֥ל בְּמִצְרַ֖יִם חִנָּ֑ם
The only free fish worth waxing nostalgic over is the herring served at a kiddush. Of course, there are other free fishes in Jewish life (e.g., lox at a bris, tuna salad at shalosh seudos) but they’re either not nostalgia inducing or are too infrequently enjoyed free to make sense in context. Granted someone makes or sponsors a kiddush so the herring isn’t really free, but it is free to eat, and that is what the complainers remembered. Careful reading of the pasuk shows that they remembered that the eating was free, not that the fish itself was free.