But I think many people are not ready for the real world that their skills lift them into: being a contractor really is being your own business, and despite all the schtick about "platforms taking care of their freelancers", it's a business relationship, and really you all need to be looking out for yourselves.
At the same time, it comes across as pretty tone deaf of Upwork, and the developer attitude is understandable as well: can you imagine that if you faced similar risk from your financial / payment / banking services (such as, Payoneer or Pay Pal freezing your funds) Upwork would agree to cover you into another account until it was resolved? Highly doubt it.
Shouldn't Upwork have insurance for these types of fraud cases so they specifically don't have to pass the cost onto devs?
You can just refuse to refund Upwork. They are giving you the collections treatment, you can tell them to stop harassing you as well as report their behavior to the competent authority in your jurisdiction or theirs. Tell them to instead chase down the money from the payer.
edit: I remember I deeply reviewed a contract for a Russian Freelancing platform that was precise and fair -- I'm quite sure it had a clause that was like, once the payment hits your bank, it's yours no matter what. But the sensible thing was they had enough other precautionary stuff in the contract about how they handled payments, disputes and clients and so on, that it looked like they could make that work. They basically covered you for any clients that tried to abscond, I think.