The 40+ years the Voyager probes have stuck around for is impressive... but the actual primary mission duration was about 12 years (for Voyager 2), and that was with two (planned but not guaranteed) extensions for Uranus and Neptune.

The RTGs on both probes have decayed A fair amount at this point and are producing a lot less power.

10 years may seem short, but combined with an electrically powered thruster there is potential for doing types of missions we have not really been able to do before. That 10 years could be spent doing propulsion.

If you want to use it more in the science phase, use chemical rockets to get up to speed and then boot up the reactor in time to say, decelerate into orbit and you are looking at having the majority of that 10 years used at the destination.

My main concern isn't really the duration, but the reliability of the moving parts. But without plutonium, there are not a whole lot of other options for powering missions to Uranus and Neptune.