Earlier quoted context omitted.
With everybody using online wallets this particular aspect of crypto is long gone.
There are over 30 million BTC addresses with a balance. So if everyone used "online wallets", they'd either have to be intentionally leaving dust in their exchange wallets, or there is some Superman 3 salami slicing going on and the exchanges have very patiently held off on sweeping up their ill-gotten gains.
I was not aware that every single exchange only has 1 wallet for all of its users?