Consider the following way of describing the black hole information paradox:
Suppose we start with a pure quantum state and a black hole of mass $M$. Now we throw the pure state into the black hole and wait until the black hole has radiated away sufficient energy to again have mass $M$. The argument (given eg. here) is that we started with a pure state and ended with a mixed state and thus the black hole has performed a non-unitary transformation on the system and information has been lost.
$\textbf{What I don't understand:}$ everything we know about quantum mechanics implies that the quantum nature of a system is extremely fragile - temperature changes, external forces, etc. can all destroy the "quantumness" of a system. Why do we expect that throwing a quantum system in a black hole is going to preserve its quantum nature? Wouldn't the expectation be that the system ceases to be described by quantum mechanics and thus one does not need to preserve unitarity and there is really no paradox at all in losing information? I feel I am misunderstanding something here...