I have heard people talk about how quantum computing could kill Bitcoin, so would Bitcoin be able to "fork off" into a quantum resistant blockchain in a kind of hard fork when the time comes?
The suggestions that quantum computing could kill bitcoin are made by people who don`t know anything about quantum computers or the blockchain.
I think that the human civilization would need decades to make quantum computers cheap and available for the majority of people.I don`t see this as a threat,perhaps this is an opportunity to improve even further all the crypto technologies.
Based on this your argument then bitcoin and blockchain is not safe because if you really follow the way technology is advancing, then you will know that the years that is being estimated to arrive at your conclusion could be fifty years early which is why there is need for further development of the blockchain technology to withstand any attack now or in the future. But I expect that Satoshi in his wisdom must have known this before concluding on bitcoin.