So, I think you read the wrong thing into that.
Fusion being 50 years away just means "we have no idea how to get there yet". The joke, which you've obviously heard, is that fusion used to be 50 years away in the 1950s, and it's still 50 years away in 2021. It's used to mean that "we don't know how to achieve it yet".
Something being 10 years away, on the other hand, means that "we know everything we need to know to get there, now it's just a matter of funding and construction".
It's not about optimism or pessimism, really. And it's not really about the actual time needed neither. In the 1940s, going to the moon was way "more than 50 years away". Then it happened at breathtaking speed.