But what if hawking radiation doesn't exist? I think it's quite a leap to assume it exists. First, Hawking's own explainations of it differ between his paper and his book. Completely different mechanistic description. So how can we pin down what the rate of radiation would be for a mass when it would depend on the mechanism.

Second, we are talking about a quantum and general relativity interaction. Those two things we famously don't know how to combine. Supposedly no one can do it. And some have tried, creating entirely speculative physic like loop-quantum gravity. Yet Hawking can make one speculation about it set in the most extreme and exceptional evironment in the universe, which we've never interacted with, and we just assume he got it right.