I am listening to the video, fast-forwarded through all the Alice->Bob stuff (although it is highly educational), and noticed Lopp said he wasn't sure satoshi ever commented on threats of quantum computing. He did indeed address the subject one time:
However, if something happened and the signatures were compromised (perhaps integer factorization is solved, quantum computers?), then even agreeing upon the last valid block would be worthless.
True, if it happened suddenly. If it happens gradually, we can still transition to something stronger. When you run the upgraded software for the first time, it would re-sign all your money with the new stronger signature algorithm. (by creating a transaction sending the money to yourself with the stronger sig)
Like most things done by the Bitcoin Core devs, I'm sure they will approach proposed changes very carefully and slowly. First, private corporations like IBM and Google need to demonstrate that they can crack ECDS or something equally difficult. By that point, hopefully core devs will have something in place to roll out in a new version to either protect or destroy vulnerable coins.
The first real-world applications of such advanced quantum computing power will assuredly be military-related, but we might not know about their existence.