Anyone who wants to connect to the bank becomes its co-owner.
This is not sounding more appropriate in general or at least for me. Because we are just participating in bitcoin ecosystem and it got nothing to do becoming one of the owner of system but we are just owning some equivalent units based on the amount of our investments. Moreover in my opinion none could own the bitcoin ecosystem neither fully nor partially.
You can always get your asset back at any time by returning its guarantees (smart contract tokens) to the bank.
I think you're not considering the volatile nature of bitcoin markets because we are not guaranteed getting our investments intact but it may be higher or lower based on market fluctuations and it is more likely how usual stock market is working hence this is not new due to bitcoins.
Judging by your posts, you do not believe in the power of Ethereum, which can change the future of the entire crypto market, I think bitcoin will not always be in the first place.