The problem I see with this mission is that it's essentially another "flags and footprints" mission on a grand scale: yes, the mission time is much longer, so

The problem I see with this mission is that it's essentially another "flags and footprints" mission on a grand scale: yes, the mission time is much longer, so perhaps there would be time to do something valid at Neptune or Saturn, but there aren't many things that could be done on such a mission that would make it cost-positive with an initial cost of $14 trillion. So what happens? We go once, can't afford to go again, and suddenly everything in the inner system is anticlimactic and the public loses interest in space for another 40 years.