Here's the logic: If space launches are expensive, you spend more on your satellite to make it worthwhile. Give it more features, etc. Then spend even more to make it extremely reliable, because replacing it would also be expensive (both the launch and the hardware itself). If launches are cheap and routine, you can just launch a cheap simple satellite for each type of sensor you want, and even if it fails you can launch backups (or just activate stand-bys already in orbit). You can even aggregate the sensor data from a swarm of smaller satellites to generate the view of a single large virtual telescope, potentially bigger than anything you could construct on the ground.