I'm not talking about solipsism, or the evil demon/genius of Descartes.
I'm talking about the McNamara fallacy, which is obviously a fallacy. Can we say that everything that exists is measurable, even if we cannot measure it a some point in time given some state of the art? That would explain the McNamara fallacy.
It is also often said that what doesn't get measured cannot be managed. But what is measured, is cheated/played with. Are managers hopeless?
Philosophy is said to be very good for critical thinking, knowledge, etc. How could we know if we could not measure the effect of learning philosophy?
If we cannot measure and objectively compare, how can we know that some perception is not the placebo effect, confirmation bias, or just a delusion of any sort? That's what control groups are for.
Finally, metrics could be wrong, as they often are, but how could we know without different and better metrics? In the McNamara example, body count may be a bad metric, but we may only be able to reliably tell so after comparing with other metrics, e.g. area under control, economic cost, etc.