[angry_octet]

Yes. Consider trialling a new drug that is supposed to prevent/cure a terrible disease, and has a well explained mechanism of effect, supported by clinical results of some % of sick people no longer being sick after treatment.

If you want an RCT you need a control group who will be subjected to the horrible disease with standard treatment (and sometimes a group with no treatment), and a group subject to the experimental treatment. Who will volunteer for this trial? Perfectly health people? Or people with the disease?

Like, maybe the measles vaccine is all just placebo effect and good sanitation and handwashing practices. Can we do an RCT where we inject some children with saline instead of vaccine, then see what happens?

Because we can't be certain the children of crunchy hipsters who don't get immunised get measles is related to lack of immunisation, it might be they expose themselves to exotic strains by taking exotic holidays.