It’s possible that you’re not conscious. So your subjective view may be correct for you. To those who are conscious, this argument doesn’t really matter, and the proof is simply in the pudding.
If we accept subjective feeling as definitive proof that something exists, that opens a Pandora’s box of entities. People have deeply held subjective beliefs about things like God, afterlife experiences, out-of-body experiences, and many others. It seems unfair to me to dismiss this kind of subjective evidence in these cases, while accepting it without question for experience of consciousness.
It’s a subjective experience argument. As a conscious person, if someone tells me they don’t believe in consciousness, then I’m inclined to believe they have a reason for saying that. They must not be experiencing consciousness the same way I am.
Interestingly, a non-trivial number of people have no internal monologue (https://www.iflscience.com/people-with-no-internal-monologue...). It would be reasonable to assume the experiential side of consciousness is on a spectrum, with extreme edge cases on both ends. It’s not unreasonable to assume that some people are barely experiencing it, and some not at all. It would certainly explain to me (someone who experiences it quite intensely) why some would claim it doesn’t exist. Because for them, it might not.
"Earth is flat" is an objective statement. "I experience consciousness" is subjective, similarly to "I am experiencing pain". If someone tells me "pain doesn't exist" while I know it exists (because I have experienced it), I can be certain that that person is wrong. Even though I can't prove it to him.
I've been thinking the same (that people who claim it doesn't exist don't have it) but it had never occurred to me that it might be on a spectrum. It actually makes perfect sense.
No conscious person can know if another person is conscious. There is no 'sensation' of experiencing another conscious. Given how many people can and have been fooled by AI, this lack of ability to sense another consciousness is clear.
It's a fair point, though. If one rejects a subjective experience without any doubt, the most plausible explanation is that they don't have that subjective experience.
If I tell you that I see numbers as colours, and you say "that's not possible", I would assume that you don't have synesthesia [1]. I wouldn't doubt the fact that I have it [2].
Similarly, if some people argue that consciousness is not something like I describe, I would naturally assume that they don't experience it the same way as I do.