Finance is a legit use case. If you think it's not then by extension you have to write off the legacy financial system as well.
Think about something like private/public pensions, annuities etc. A cogitating being knows that they are getting older and will need money for retirement, but they also should know that there are essentially just 4 ways to make an investment grow without any labor input from the investor: the bond market, the stock market, the real estate market and private lending to companies or individuals.
Now with the possible exception of private lending a citizen can do all the above, and remember they also know that they will need money for retirement. So tell me again why do they need public/private pensions and annuities? And why we allow such actors to operate given they redundacy considering the fact that they are preying on people's fears that they won't be able to make it in the future?
If crypto preys on people's greed then the legacy financial system preys on people's fears. They are 2 equally bad behaviors.