MHO is that for most people, hair thins because it's old and worn out. Even people who wear buns 24/7 (Wanakee) have to dust regularly to keep ends looking thick and full. There are those who can get away with not doing so, but IMO most won't.
I don't agree with the theory that ends look thin because they are at different growing stages because if that were the case, then when you have a TWA, you'd start looking like a troll before too long because the different growth stages would not wait till a couple of years later to become evident. They should be evident right from the get-go. But that's not usually the case.
Hair thins because ends that have been around a while split and then tear away, leaving a partial hair that has a smaller diameter than the rest of the hair. I bet you if you were to follow the hair from ends to base, you'd notice it gets thicker as you get closer to your scalp because it is only the newer hair that is whole.
Many disagree with me but the only solution is to dust. Search and destroy means waiting till damage has been done to then chop off the damaged hair. Regular dusting means you prevent that damage from ever coming to be by getting rid of the hint that it's about to happen. Dusting means getting rid of a lot less than you would waiting for months for the hair to be already in bad shape.