Ahhh, the terminal length discussion.
Hair has a life cycle, consisting of three separate stages. Growth, Resting, Dead.
Shed hair is hair that has reached the dead stage of the cycle. It dies, and it is released from the follicle, to make room for the next strand of hair. You can tell that your leg, arm, eyebrow, and eyelash hair all have a terminal length, because even if you don't shave your legs for three years, the hair on your legs isn't going to get much longer than 1/2 inch. It dies, and it is shed, and a new hair shows up.
Hair on your head does the same thing, and you have to remember, we are talking on a strand by strand level, and every strand is not on the same 'schedule'. If it was, we would molt/shed like the furred and feathered creatures do.
So. You have a strand of hair that starts growing. It grows for say - a year. Then, it rests for 6 months - not growing, but not dead, either. Then, it starts growing again. This cycle can go back and forth, back and forth, for
years. At some point though, that strand of hair IS going to die. When that happens, it's shed, and a brand new strand, on it's own new cycle, starts to grow. This is why we shed so much hair a day - there are anything between 100K and 250K strands of hair on your head - and about 100 of them 'die' daily.
That ties into terminal length, because it is the maximum length that your hair can possibly achieve if it is never cut from the day it is 'born' (in the follicle of a shed hair) until the day it dies, and sheds. We can't calculate our terminal length though, because there is no known way to determine how long gap between birth and death of a hair is.
And to be honest, this REALLY is a topic that you should have done a search on - there are multiple threads here on the same topic - and it's not a matter of belief. That's like saying "I don't believe people die"
It happens, whether you agree with it or believe it or not. And if you don't understand what's actually happening, then what you think you don't believe isn't what's actually going on, anyway.