This reminds me of a homebrew cantrip that I call "Elfshot", that derives from British folkbelief about elfshot.
Compare how the Shillelagh cantrip uses a normal simple melee weapon (club or staff), but applies the spellcasting ability instead of Strength. A cantrip can interact with a normal weapon.
In this case, the weapon is effectively like a "requirement" (or if understood differently a "trigger") in order to cast the spell.
For the Elfshot cantrip uses the spellcasting ability, but the caster must hold and use a bow weapon. Now, the cantrip caster shoots energy arrows instead of actual arrows. (It is ok to visualize this like the Ranger using an energy bow in the D&D cartoon.) These spirit arrows are invisible, and deal psychic damage (from pain, and the feelings of "pins and needles"), instead of piercing damage. Then, when the target reduces to zero hit points, the target suffers a "stroke" instead of dying.
Anyway, cantrips are a versatile format to do all kinds of at-will features, even while using a normal weapon.
It makes sense to design nonmagical at-will features for martial classes, using the cantrip format.