I really don't understand the purpose. If someone wants to buy and sell bitcoins, they can do it directly and digitally. If the ETF is for bags of corn or tons of copper, those can't easily be bought and sold because of their physical attributes, but bitcoins don't have this limitation.
They do. What characterises buying bags of corn or tons of copper is that it's
impractical, not that it's physical. For many people (in fact >99% of them) buying BTC is the same.
Now you might say that it's easy, but it's easy for
you. A farmer would say buying bags of corn is easy. A copper smelter would say buying tons of copper is easy.. it's really about the majority of people not the minority.