Sylvan Safekeeper protects key creatures like Phyrexian Dreadnought and Hermit Druid from Swords to Plowshares, since sacrificing a land gives the targeted creature shroud and the removal fizzles. It does nothing against Armageddon, which destroys lands without targeting creatures, so the land you sacrifice is a real cost to weigh against Terrageddon's land destruction. Stifle is weaker here because Terrageddon's key cards (Armageddon, Terravore, Swords to Plowshares) are spells, which it can't counter. It can still hit activations like Wasteland or Weathered Wayfarer, but that is not enough to justify all three slots. Keep the core combo plan intact and prioritize resolving a fast Reanimate or Shallow Grave threat before they can Armageddon you back to parity.