But it's not. You can do an RCT of the new treatment vs the old treatment. You won't get a direct measure of its absolute efficacy but you will know if it's sup

But it's not. You can do an RCT of the new treatment vs the old treatment. You won't get a direct measure of its absolute efficacy but you will know if it's superior/ non-inferior to the best known thing. And then you can use observational techniques to estimate the absolute values. That's exactly what you would do if you wanted to develop, say, a new flu vaccine that you thought would outperform current vaccines. You get the most important information: whether or not we should switch to the new one. If you have a new vaccine for a disease for which there is no existing vaccine you do a standard placebo controlled RCT which gives you a direct, high quality measurement of efficacy and side effects.