Damping Sphere isn't good. It's a prison piece in a deck that doesn't particularly want to prison.
There are a few real problems with the deck as constructed.
First, you're a little softer to Jeskai now, as everyone and their mother has maindeck ways to kill artifacts. You used be be able to say "my sword will survive if it's not Dack'd" with high certainty, and now you can't.
Second, Inferno Titan Oath is a bigger part of the metagame. Stoneblade can beat normal Griselbrand Oath, albeit with some difficulty. You can never in a million years beat Inferno Titan Oath. You just don't have the tools to do that efficiently, and you're too aggressive of a deck to simply play control until you find Mentor.
Third, the outcomes decks have homogenized into a cleaner variant than before. You used to be roughly favored against people trying to really durdle a lot with outcomes->Tendrils. Now you can just die to the secondary effect. You're still probably favored, but it's by less than before, and it's less clean than just casting Stony Silence.
Basically, the metagame changed. This deck attacked Shops decks and trash deck (White Hate/Merfolk/Humans/Other) players better than a lot of the rest of the metagame, while still being effective enough to get wins against Jeskai and Outcomes if you could outplay your opponent. Now that you don't have as much trash to take out (we're a ways away from Champs, for example, and the metagame has consolidated), and the good decks are better against you. So...you end up with a deck that's fundamentally a metagame call against a meta that no longer exists.