The said argument doesn't give certainties, it only gives you chances of something happening in the next 50,000 years based on what happened in the past - the chance correctly being extremely low.

Chance of event more extreme than anything ever happened before depends on your sample size. If your reference class is tiny, you need to assign high probability to extreme events; if your class is huge, probability of an extreme event is low. (The main complication is that samples are almost never close to being independent, and figuring out exact numbers is really difficult in practice. I'm not going to get into this, there might be some estimation method for that based on meta-reference-classes.)