It's time to stop jetting around the solar system on chemical rockets. Designers and funding should be directed towards lofting and running multi-megawatt reactors. They would be used to power multiple ION engines
Yes... let's develop heavy power sources in order to power weak propulsion systems - what a great idea! Multiple ones aren't much better, you still need to power them, and you have to multiply a small number (thrust per engine) by dozens (or more) to get a usefully large number (thrust) for any significant spacecraft. (Which will still be far short, by orders of magnitude, for a useful size for a manned mission.)
Their extraordinary ISP is very attractive from an academic point of view and when considered in isolation... but real world vehicles aren't academic and the engines are but one part of the whole vehicle. When you start to design an actual vehicle and an actual mission, their extraordinarily low thrust-to-weight ratio precludes them from being useful except in a few very specific circumstances.