"You can have bits of your visual field, for instance, that you're not introspectively aware of but are part of your consciousness" Maybe, but in the current context this is basically begging the question, whereas I've at least sketched an argument (albeit one you can probably resist without catastrophic cost).
EDIT: Strictly speaking, I don't think people with the Dennettian view have to or should deny that there is phenomenally conscious content that isn't in fact introspectively accessed. What they do/should deny is that there is p-conscious content that you couldn't access even if you tried.
I've seen Dan Dennett (in effect) argue for it as follows: if a human adult subject reports NOT experiencing something in a lab experiment and we're sure they're sincere, and that they were paying attention to what they were experiencing, that is immediately pretty much 100% proof that they are not having a conscious experience of that thing, no matter what is going on in the purely perceptual (functional) regions of their brains and how much it resembles typical cases of a conscious experience of that thing. The best explanation for this is that its just part of our concept of "conscious" that a conscious experience is one that you're (at least potentially) introspectively aware that you're having. Indeed (my point not Dennett's), this is how we found out that there is such a thing as "unconscious perception", we found out that information about external things can get into the brain through the eye, without the person being aware that that information is there. If we don't think that conscious experiences are ones you're (at least potentially) introspectively aware of having, it's not clear why this would be evidence for the existence of unconscious perception. But almost all consciousness scientists and philosophers of mind accept that unconscious perception can happen.
Here's Dennett (from a paper co-authored with someone else) in his own words on this, critiquing a particular neuroscientific theory of consciousness:
"It is easy to imagine what a conversation would sound like between F&L and a patient (P) whose access to the locally recurrent activity for color was somehow surgically removed. F&L: âYou are conscious of the redness of the apple.â P: âI am? I donât see any color. It just looks grey. Why do you think Iâm consciously experiencing red?â F&L: âBecause we can detect recurrent processing in color areas in your visual cortex.â P: âBut I really donât see any color. I see the apple, but nothing colored. Yet you still insist that I am conscious of the color red?â F&L: âYes, because local recurrency correlates with conscious awareness.â P: âDoesnât it mean something that I am telling you Iâm not experiencing red at all? Doesnât that suggest local recurrency itself isnât sufficient for conscious awareness?"
I don't personally endorse Dennett's view on this, I give to animal causes, and I think it is a big mistake to be so sure of it that you ignore the risk of animal suffering entirely, plus I don't think we can just assume that animals can't be introspectively aware of their own experiences. But I don't think the view itself is crazy or inexplicable, and I have moderate credence (25% maybe?) that it is correct.
Just the stuff I already said about the success he seems to have had. It is also true that many people hate him and think he's ridiculous, but I think that makes him polarizing rather than disastrous. I suppose you could phrase it as "he was a disaster in some ways but a success in others" if you want to.
The thing about Yudkowsky is that, yes, on the one hand, every time I read him, I think he surely must be coming across as super-weird and dodgy to "normal" people. But on the other hand, actually, it seems like he HAS done really well in getting people to take his ideas seriously? Sam Altman was trolling Yudkowsky on twitter a while back about how many of the people running/founding AGI labs had been inspired to do so by his work. He got invited to write on AI governance for TIME despite having no formal qualifications or significant scientific achievements whatsoever. I think if we actually look at his track record, he has done pretty well at convincing influential people to adopt what were once extremely fringe views, whilst also succeeding in being seen by the wider world as one of the most important proponents of those views, despite an almost complete lack of mainstream, legible credentials.
People who deny animal consciousness are often working with a background assumption that any thing can in principle be perceived unconsciously, and that in practice loads of unconscious representation goes on in the human brain. It's not clear what use a conscious pain is above a unconscious perception of bodily damage.