The beauty of bitcoin is that it can be easily self-custodied, making ETFs and banks unnecessary. And when anyone does prefer to have a bank or other institution hold their bitcoin, the institution will only ever be able to offer bitcoin IOUs, which have an unlimited supply, and not real bitcoin, which is perfectly scarce.
So even in that situation, the amount of real bitcoin is fixed at 20,999,999.9769, and will never be a single sat more.