Nobody in their right mind is gonna Downfall that Dragon before it triggers the most detrimental ability in the entire game that isn't Final Fortune. Just take 4, then they discard, then kill it so they have nothing in their hand.
Low cost aggressive decks don't want a 4 drop, especially not one that makes it so you permanently can't play whatever is in your hand. This card is one of the most garbage mythics in the last couple of years, honestly.
I don't know that I agree with you. That thing could come down when the player across the board is in dire straights. Imagine this scenario with Haste.
T1 - Swiftspear, Probe, 18
T2 - Bolt, Bolt-variant, Swiftspear swing, 9
T3 - Skullcrack, Swiftspear, 4
T4 - Dragon, Swiftspear, -1
What do you do in that situation? For RDW or its ilk, that's a reasonable line you might see. You're going to have to cast removal or have answers at some point. The problem is now it's not even a situation that can happen because the card is garbage.
I don't really need much to run it. Sidisi, Risen Executioner, and Gurmag Angler are good but it'd be nice to see a solid Zombie on the lower end of the curve. Not really sure Shambling Ghoul is worth an inclusion. I wasn't really expecting it to be a competitive archetype but I thought it'd be fun to run with the same them for my EDH and Standard decks while Whip is still in Standard.
What about Meandering Towershell?