There are several differences between these old predictions and now: 1. We're now assuming based on the limits of known physics rather than the expected limits

There are several differences between these old predictions and now: 1. We're now assuming based on the limits of known physics rather than the expected limits of technology. My father told me he once read a book that said it was impossible to go to the Moon because it would take 750 years to get there. The speed of light was known at that time but assumptions were made about the limits of acceleration and velocity that weren't grounded in science; 2. Things like the rocket equation are important. Specifically, the reaction mass problem. Even more specifically, the only way for an independent body to accelerate in space that we know of is by expelling mass (or energy). You need to carry that mass and this quickly becomes inefficient. A good example of this is how people have recently started to say "if we just accelerate at 1G we can get to Alpha Centauri in a few years". 1G acceleration sounds easy but it's not. So how would you travel between stars? I think it most likely that you'd use a ton of energy to accelerate (or decelerate) a vessel with lasers on a solar sail. Why? Because energy from a star is abundant and cheap and it doesn't require carrying reaction mass. If you accept that, then the need for Dyson Swarms becomes more obvious. 3. Breaking the 2 assumptions I listed makes spacefaring life more common not less. By this I mean we only really need to consider a million light years (of light cone) because of the speed of light. If FTL or even time travel were possible, then you need to consider a volume of spacetime orders of magnitude larger.