I didn't realize this would trigger on artifacts and enchantments too until I heard it discussed in Lucky Paper Radio.
I have mixed feelings about the 1/5 body of Murmuring Mystic, but for now I think I'm ok with keeping it, as being a 4-drop that starts triggering T5 is already a slow card that doesn't pressure aggro. A 4 mana 3/2 is even worse under pressure.
Another reason I'm keeping Mystic is that build arounds should not be fragile.
There is a big difference in triggering on everything though: it's much less awkward with other payoffs like The Mirari Conjecture in a spells deck, and is playable in a broader range of low-creature decks. Another advantage (that Andy really cares about) is that it uses a much more common token.
Anyone else gave consideration to making this swap?