Doctor Doom
The indestructibility clause is a conditional that Doom has to earn on the way down, not one he arrives holding. Because the two Doombot tokens come from an enters trigger rather than a static effect, there is a real window: if you control no other artifact creature or Plan when he resolves, a destroy-based removal spell aimed at the trigger kills a plain 3/3 before the robots ever exist. Clear that gate (or land him into a board that already has an artifact creature or Plan online) and the equation flips hard. He becomes a self-referencing engine: each Doombot satisfies the "as long as you control an artifact creature" clause, so an opponent must unwind both bodies before destroy-effects even read on him, or route around indestructibility entirely with exile, bounce, or minus-toughness. The Plan clause widens the same escape hatch for decks built to keep one active.
The end-step trigger is the tax that pays for the durability. A card every turn with no cost but a single life is a rate that would be alarming on a fragile body; on one this hard to answer through conventional removal, the life loss is the only clock running against you, and it runs slowly. The design hands a six-mana legend a persistent draw engine and near-immunity to removal without tipping into oppression by making you bleed yourself throughout, and by leaving the armor removable in one beat: strip the artifacts or Plans and he is a 3/3 again.
