Most trials like this have interim reviews for exactly this reason. They take unfinished data at intervals - say, if it’s a 5-year study, they may look at 1 year in and so on - to see if the unknown treatment has already reached a statistically significant improvement over the standard of care (the usual control). If it has, the trial can be stopped early, and all the controls switched to the new med (or conversely if the test is significantly worse they can be switched back to standard treatment).

That way, you first ensure that everyone in your trial is getting at least the current best treatment, and are able to switch over as fast as possible.

It’s pretty unusual for new treatments to be that significantly better; incremental improvement is the norm (but like compound interest, small improvements every year for 50 years can lead to the dramatic improvement in, say, many cancer treatments that we see).

If you’re wondering why we even bother with clinical trials when we know something is going to be da bomb, we are really good at “knowing” wrong - even scientists deep in the field often have incorrect expectations. It’s probably much more common to stop trials early because the test med is worse than standard of care, than because they’re even better than expected.