> if they're genuinely out of research ideas to test in space

A bigger problem is lack of expertise. Astronauts are not specialist in whatever is the topic of the current experiment. You need probably like 5 years of training (assume the second half of the undergraduate degree, and perhaps the first half of the PhD). So experiments must be fully automated except for a button to turn they on and off.

Lots of research has technicians doing the actual experimental tasks, your argument would benefit from even a short list of experiments that have not been done because astronauts couldn't be expected to handle it.

We don't really need to send "astronauts"(highly trained operatives) to space anymore. SpaceX has made that happen.