Here's an idea: instead of spending all this money now to launch probes from Earth, why not spend it instead on building a base with launch infrastructure on the Moon? No atmosphere, no environment to worry about, lesser gravity well...the list of advantages is quite large. The only disadvantage is it would take a while to get going. But the same could be said for the space industry 50 years ago. So we could spend a lot of money on a lunar base now and get huge payoffs later, or keep spending almost as much on Earth-launched probes for the next several decades and advance the human presence in space not one whit.
NASA still hasn't figured this out. The public is not *interested* in these pure science missions, regardless of how beneficial they are to scientists and engineers. The public wants the glory, grandeur, and *adventure* of Apollo. And without public backing, NASA's budget gets whacked again and again and again. NASA needs to come up with things that capture the public's imagination like the glory days of the 1960's. Then they'll get the money and political clout to do big things. I'm sure most American's don't give two damns about a mission to Uranus or Neptune.