I can figure out a partial answer:
There was a decline in launches following the end of the spaceshuttle program that we can see here:
From 2000 to 2015 the launch rate was 50% of what it was in the 80's.

Missions have focused on quality and advancement of sensors and equipment, rather than a higher quantity of less sophisticated missions.
The price of launches is perhaps going to change that soon. A launch in 1981 cost 99 times more than in 2020:

Compared to the 1980's the CAD design suites, sensor prices, booster technologies, orbital mechanics models, N-body simulations, should contribute to a higher productivity rate, lower design price and facilitate slingshots and other complex mission physics equations.
However, the specialized nature of the extreme equipment used for space exploration and the cost of the skilled workforce counterbalances those effects.
Considering the recent leaps in launch economy, and new space organizations in India, Japan and China, perhaps we will see a higher throughput of lower budget missions soon, perhaps these less well known targets:
- Europa, Icy Ocean
- Uranus, Not investigated nearby since Voyager 2
- Neptune, Also never visited since Voyager 2
- Enceladus, Active Geysers
- Titan, Hydrocarbon Lakes
- Triton, Retrograde Orbit
- Ceres, Water Ice
- Charon, Geological Complexity
- Vesta, Diverse Geology
- Eris, Distant Dwarf
- Pluto, very complex hydrocarbon glaciers and cryovolcanos