No one knows that it is not measurable. Is love measurable? is hunger? happiness? they seem to be as measurable as consciousness, that is to say we can at least measure them in binary as either present or not.
If you accept the shared reality on the other hand, consciousness is measurable to some degree. So the real question is do you or do you not accept we share experiences?
As an addendum, if it is all up to me as you suggested, I just made consciousness measurable so there is no need to keep arguing about it.
Far from it. There could very well be an external world, and one populated by plenty of other and fully real human beings even, but your own personal view or understanding of it could be distorted or false.
This could be simply because you're hallucinating, or insane, or your brain could be injured, or could be living in a virtual reality (which itself exists in some other "real" reality), or you could be the proverbial brain in a vat, or aliens (or god or a demon/devil) could be deceiving you, etc.
But that's not what solipsism typically means. Solipsism usually refers to the position that only you (or perhaps only your own mind) exist. By that commonly accepted definition, one could be mistaken or deceived in any of the ways I laid out earlier without them entailing solipsism.
And in the "shared reality" option, consciousness is somewhat measurable.