It's not about mining or fixed amount of coins that can be mined.
Quantum computers can (potentially) break underlying cryptography so badly that they can reveal private key of any Bitcoin address just from the public address.
Due to this someday there will need to be a major hard-fork to combat it, ideally before quantum computers mature enough to handle such tasks.
The good thing about Monero is that no one knows your address (unless you publish it) and amount, so quantum computer would need to generate all possible addresses AND sync all of them starting from block #1
because you can't just lookup the amount and address on blockchain like in Bitcoin.
This task even for quantum computers is impossible because the biggest bottleneck is the memory speed and not computing speed.
Even if you calculated theoretical numbers that are physically out of the reach for silicon, it would take too long to sync all the addresses.
Monero developers are actively testing quantum-resistant algorithms to make Monero more secure, hopefully to be deployed in time.
This is interesting (two years old video):
Adam Corbo, a Physicist and Quantum Computing enthusiast on Monero Post-Quantum – #Monerotopia22 If that is true and really possible, and if quantum computers will be publicly available in the next decade, then the crypto market will be destroyed. If Bitcoin, the second most secure currency, isn't secured anymore, then it will likely lose half of its value in the first year of the existence of those computers, and it will be the 90s and 2000s once again. Imagine someone developing an organized phishing program on these computers to steal Bitcoins.
If I remember correctly, in the 90s, there was a virus developed by a Filipino in some poor area of the country. The man was able to steal personal information via that virus and sell it. It's scary to think that happened in a poor country at a time when only the richest 10% could afford such a device. It will happen once these computers become public, and it will be a disaster unless someone creates a new improved concept of blockchain.