How would you say this assessment is wrong?
Finance firms provide lots of very sophisticated ways for money to get from people who have lots of money that they don't currently have a use for, into the hands of people who have a need for money they don't currently have.
Note this is a weak claim. I'm not saying anything about the efficiency of this process or whether the financial industry is good or evil or anything. But you can't criticize the finance industry without at least understanding and engaging its outputs.
The argument I would like to make is not that finance is bad intrinsically - as you've said above. The real problem is that the current instantiation of this concept has grown a series of goals of its own, and to fulfill those goals it is doing things that use the common, shared, limited pool of resources of the global economy, siphoning off stuff that the rest of us could put to better use.
In other words, they could perform useful services for us, and they do. It's all the extra layer on top of that that's the problem.
In a strict, mathematical sense, the OP probably exaggerated when saying they are entirely parasitical.
The financial industry is as evil as any profit oriented industry can be and to single it out as something completely wasteful and parasitic is unreasonable.