Nothing happens to them, they just stay in the wallet forever, or until they invent a quantum computer that can unlock the wallet.
Quantum will not so easy to open it. i guess any SHA already had quantum resistance. you need to understand what the quantum computer is and how it works. I guess it will degrade the SHA that used by crypto right now.
I am sure that in the next 10 years, the cryptocurrency market should not be afraid of new computing technologies. The danger is represented by economic methods of market manipulation.