If you build an experiment (at any scale) to test out an idea that you think is going to work, and it doesn't work, you made progress. Now you know something really essential -- that thing that seemed like a well-motivated idea has some kind of unknown flaw. With that in hand, you can proceed to try something better, whether it is a refined approach or something entirely new.
Without doing the experiment, that well-motivated idea would still look like something worth trying.
If a large team of people were willing to spend years and lots of resources on an Alzheimer's clinical trial, it was likely to be because they thought it had a real chance of working. If it didn't work, we all benefit from their effort.