Its obviously a much less powerful card overall, and you cant just throw it into a deck with no concern for what else is in there, but its possible there is a deck built with The Cauldron of Eternity in mind and then this deck could want Gray Merchant, but it really has to be a dedicated self-mill deck for it to work, because if you cast Gray Merchant and it dies naturally (which is the most common play pattern), it doesnt work with the Cauldron unless the Cauldron isnt on the battlefield yet
Your deck did exactly what it was supposed to do, and you came out on top
Whisperwood Elemental: Given the short lifespan of creatures in some environments, Im surprised that Whisperwood Elemental hasnt seen even more play
Will it be one of the intended combinations, or something unexpected
Fill up on cheap creatures and token-makers then toss out a lot of targeting cantrips and buffs, like Ancestral Anger and Sudden Breakthrough