The voyager probes only got as far as they did because of their trajectory that got some massive (and rare) slingshots, it will take ages for the new horizons probe to get anywhere near as far.
We could probably spam missions to some other planets, who will pay for it though? We are not at the stage where an 'out of the box’s mission can do that I think?
Maybe I could have been more explicit. Without the planetary alignment that made the voyager probes possible an equivalent mission would be ridiculously expensive/impossible due to the fuel requirements (and wouldn’t be able to visit all of the planets)
If starship/new glen/the rocket lab one work, it might become more feasible.
Instead, sending smaller, simpler probes that just visit one planet/moon would be much more cost effective, but still expensive.
We have already got a lot of the low hanging planetary science fruit from existing missions. New missions would need new/novel sensors or need Landers/aircraft which make them much more expensive.
Even just a ‘standard’ interplanetary mission isn’t just an out of the box job like current earth satalites are becoming.