It was impossible in the sense that nobody else did it before. It was not impossible as in you need to violate basic laws of Physics or elementary Economics.

Before reusable rockets, the idea made sense. Building a rocket is expensive; if we reuse we don’t have to keep spending that money. Fundamentally, rockets are rockets. It’s not like they invented anti-gravity or anything.

It’s like climbing the Everest. Before it was done, it was still something people could plan and prepare for. But you’re not going to climb all the way to the moon, even with oxygen bottles. It’s a completely different problem to solve.

The most difficult point to argue against for people who want to defend Musk’s delusions is simple economics: at the end of the day, when you’ve solved

- the energy source problem (difficult but probably doable);

- the radiation-resistant chips issue (we know we can do it, but the resulting chip is not going be anywhere near as fast as normal GPUs on Earth);

- the head dissipation problem (physically implausible, to be charitable, with current GPUs, but considering that a space-GPU would have a fraction of the power, it would just be very difficult);

- the satellite-to-satellite communication issue, because you cannot put the equivalent of a rack on a satellite, so you’d need communication to be more useful than a couple of GeForces (sure, lasers, but then that’s additional moving parts, it’s probably doable but still a bit of work);

- the logistics to send 1 million satellites (LOL is all I can say, that’s a fair number of orders of magnitude larger than what we can do, and a hell of a lot of energy to do it);

- and all the other tiny details, such as materials and logistics just to build the thing.

Then, you still end up with something which is orders of magnitude worse and orders of magnitude more expensive than what we can already do today on Earth. There is no upside.