You’re mixing up two different things. One is normal brain development, and the other is consciousness itself. It’s true that a human being needs certain external stimuli for the brain to develop in a healthy way and for mental experience to be normal and stable. But that does not mean that, without those stimuli, consciousness does not exist.
A person can still be conscious and yet suffer severe cognitive, emotional, or behavioral changes because of sensory deprivation or an extremely limited environment. What changes is the quality of development, mental functioning, and maybe the kind of consciousness they have, not necessarily the existence of consciousness itself.
Also, there is no clear way to measure how much, how little, or whether someone has “no” consciousness at all. Because of that, claiming that a lack of stimuli means a lack of consciousness goes beyond what can actually be established.
So your argument seems to confuse “without stimuli, there is no normal development” with “without stimuli, there is no consciousness.” Those are not the same thing.
Yes, I have to admit that I don’t have any example of consciousness in an individual completely without external stimuli. In fact, I agree with you on the last point you made. My point is that we need to be more precise about what we mean by “external stimuli” exactly.
We could even say that it may be impossible to isolate the mind completely from all stimulation in the first place. That leads to the real issue: perhaps those external stimuli are always present in some form, even if they are extremely minimal, indirect, or invisible to us at first glance.
So I correct myself, the argument is not necessarily that consciousness can exist with literally no input whatsoever, but rather that what we call “external stimuli” may include things so subtle that they are easy to overlook. In that sense, the claim becomes less about the absence of stimulation and more about how little stimulation is actually enough to sustain consciousness or mental activity.