But the thing is, the Voyager project came about in a much more stable period for the US - and in a more optimistic cultural climate (we could say similar for the Apollo project which wasn't that much earlier). When we used to prioritize spending on basic science and projects like this that basically had no ROI (NASA didn't even much think in those terms back in the 70s). Now we're in a very different place where, in the US anyway, we're very pessimistic about the future. To create a Voyager project you have to have some hope, like you said "with the assumption that that future would exist and might care" - now we're in a very different place where people don't have a lot of hope about the future. And it's also different in that we now ask "what's the payback going to be?" - everything now seems to need to pay it's way.

Not saying that other countries won't be able to do stuff like this - probably China is going to take the position that the US used to hold for this kind of exploration. It seems to be a more optimistic culture at this point, but hard to say how long that lasts.