Looking at how Blue Origin is doing, it seems that that's still true.

I don't think anyone (not insane) actually doubted we could reach the moon, they were just unsure how much money it'd take per launch, and if it'd ever be worth it. Because we had already reached orbit, "add more fuel" is essentially a guaranteed success, if infinite money is allowed - at worst you do it in multiple launches and join up the parts in space. 2, 10, 1000, it'll eventually succeed, there's no reason to believe it wouldn't.

Reaching another star? Definitely more expensive, but entirely feasible if we all got our shit together and decided it was going to happen.

Medical stuff though? Humans are complicated, and there are practically no guaranteed routes, regardless of money (currently).

That's not a reason. That's an anecdote, a post selected and cherry picked example. Is this truly how you'd allocate resources?

A more correct way to use reason is to look at all possible things that have been proposed that cost $500m, and see how those turn out in general. Then possibly weight using expert knowledge about this particular problem and the possible benefits. Then consider what the opportunity cost is, i.e., what you give up by spending $500m here instead of something else.

Trite armchair rebuttals are not convincing.