Ethical banking, presumably charging low interest rates and on favorable terms, can be sustainable if the banks are willing to accept lower profit margins. Aggressive banking, almost by definition, can give better returns to the banks engaging in it, but then there is a social cost. Ethical banking, if done on a small scale, can sustain itself in spite of lower returns.
Yes, there is a sustainable future in ethical banking, because ultimately, the bank will end up with more profits. If a bank takes advantage of people, eventually they will have nothing because the people will go bankrupt. Sustainable practices will ultimately lead to the most profit, because people will not be mistreated.
Many Americans are sick of unreasonable fees, deceptive practices, and the flat-out dishonesty of banks. As a result, a new direction in banking could attract these disgruntled customers. For example, around the time of the 2008 economic meltdown, there was a great surge of interest in credit unions. Something that still earns its backers money but behaves in a more responsible fashion could really revolutionize the field.
Capital is power. Currently, the vast majority of home owners would not have become such if not for a loan. The same could be said for the vast majority of college grads, business owners, tech startups, etc. Not only does ethical banking have a future. It's the way to the future.