I'm not actually sure I would agree on not being able to store recallable memories being end of consciousness. Alzheimer and other medical conditions might very well result being unable to make new memories, but processing and reacting to stimuli based on old memories is still possible.
Reasonably I think stop of consciousness is both not storing and not reacting to.
I'd say Alzheimer's is essentially the inability to lay down memories that can be recalled beyond a very short period of time (in extreme cases, seconds).
But once your brain can no longer store ANY recallable memories, you're unconscious.
Sure, I would say it happens quite a lot (certainly while we're infants).
My only point is that to be conscious is tautologically to know that one is conscious. And knowledge is dependent on having (= being able to recall) a memory of something. Yes, colloquially there are instinctual forms of 'knowledge' where you get some vague sense something must be true (despite having no information to determine one way or another), but I'd suggest that's probably a misuse of the term.
Reasonably I think stop of consciousness is both not storing and not reacting to.