But is has already been done. An actual viable plan to get to Mars would be a new exploration, but no one has ever been willing to put up the cash for that.
Hold on, now. While I made mention of the Apollo 11 landing in my post there, my opinions on manned spaceflight in general should not be compartmentalized in the Apollo program. There are plenty of unexplored possibilities in manned spaceflight that go beyond simple (and I use the term "simple" loosely) moon landings.
But it was a colossal scientific failure.
Well, I would argue that not all things require a scientific or monetary purpose to avoid being "pointless." By your reply it sounds like you were impressed by the technological feat and awed by the accomplishment, at the very least. That hardly sounds "pointless" to me, if the program captured the hearts and minds of the American public (to say nothing of the rest of the world).
The one contribution it has made - fixing the Hubble - could have been finessed more cheaply and effectively simply by building and launching more Hubbles.
NASA was under extreme pressure to fix the current Hubble to avoid the political fallout of its initial failure, so time was a factor in their decision to repair instead of relaunch. It took a long time to build Hubble in the first place; if they'd taken the time to assemble another one, they'd miss their window to fix the mistake in the public's eye and regain their trust; failing to do that would almost certainly have cost them funding in the future. I'd say that from NASA's political standpoint, they made the right call. Unfortunately, the correct political decisions aren't always the most financially sound.