My mum's in mid stage dementia now, but we still have happy moments. Would I want her to have full use of her faculties - of course, and it is a tragedy that anyone should go through this. But she's still there, doesn't know I'm me, of course, but even saying that isn't quite right.
She still enjoys having a quick chat and a smile with people we meet on our walks. Longer chats would be a struggle. And there are definitely challenging times too! Just not all the time.
When it comes to the end stage, maybe that's a different matter - bedbound, not able to communicate, one wouldn't want that to go on for too long.