So much ignorance about this. The FBI needs its evidence to be admissible. I'm sure the FBI has no trouble accessing phones with warrantless surveillance when they need to for national security, but they need surveillance to be legal when it's used as evidence in a judicial context

No, this was for a locked iPhone. They were after encrypted data-at-rest. Warantless surveillamce would not apply, as they had a warrant - I believe - seeing as they went after the iCloud data and said it was not helpful (No backup stored there).

Additionally, an OIG report seems to suggest they did not attempt their usual avenue for acquiring access tools in order to get in, instead trying to force a legal precedent.

It always looked to me like they just wanted the tool, and the whole “can’t unlock” was just an excuse.