An assembly line of probes would be amazing. Many have said we should built a dozen copies of Kepler space telescope to then detect millions of planets. Sadly,

An assembly line of probes would be amazing. Many have said we should built a dozen copies of Kepler space telescope to then detect millions of planets. Sadly, that just isn't what the scientific community wants. Each probe is bespoke. Some parts are standardized but the actual instruments tend to be designed for one specific mission. As for testing in leo, that isn't as useful as one would think. Space in LEO is very different in terms of temperature/light/radiation. LEO orbit is much warmer and dynamic (rapid orbits) than say deep space on the way to Jupiter. So it is better to test sensors on the ground in conditions that better match those where the probe is actually going. LEO is a good vacuum, but most probes wouldn't feel any difference between real space and a vacuum chamber on earth.