It sounds like common sense, I guess, but has that ever worked, actually?
Similar "prevention" rationale is offered for governments to spy on virtually all telecom all the time, now. But this shooting happened anyway.
1) Anyone with a plan promised to stop all terrorist attacks is lying to you, stupid, or both. You can't have a free society and a 0% chance of political violence.
2) Yes, searching the possessions and communications of dead terrorists unsurprisingly are substantially more likely to lead to useful criminal leads than reading your metadata. A warrant to read this person's stuff isn't unreasonable in the slightest, an order forcing apple to do shit might be but that's a procedural thing unrelated to the core issue of "is there a good reason to read this person's stuff"