Double engine failure was confirmed, not disproven. RAT deployment was confirmed, not disproved. Pilot error, confirmed, not disproven. Preliminary and final aviation reports are mostly guesses.
If we decided to pin all aviation incidents on pilot errors, we wouldn't even have invented checklists (what do you mean you forgot, try harder the next time).
"Natural" pilot errors lead to lessons that can be incorporated into design/best practices. That does not seem to be the case given current understanding: no flaw in any switch design seems apparent, and it does not sound like something you could do by accident.
So "pilot error" is not the "cracking the case"-grade conclusion it is being made out to be, it is an act of investigative resignation. In the days following the crash, allegations of mixing up flaps and landing gear were floated, and they all turned out to be wrong. This is not even accounting for the fact that the pilots are not around to plead their case, and basic human dignity requires us to defend their case until evidence clearly points a certain way