I suspect that even if Botchtower tracks people who leave, I am not sure they'd publish those facts. It probably doesn't look good.
With a large majority of the industrialized world's people aging, there is quite a number who will become too old to witness and will die off in the next 20 years.
My Father is one of these people. I have no idea what his official status is as a JW. I suspect it is 'inactive'. I guarantee they've removed him as a Ministerial Servant. He no longer has the ability to witness. Dementia has a way of doing that to a person.