An interesting moral query: how much debt erased is worth a prison sentence of X years?

No, which is why I mentioned tor.

In case anyone feels like doing something like that, this talk is worth a listen:

https://www.youtube.com/watch?v=eQ2OZKitRwc

A talk on how Tor users got caught. In a nutshell: it wasn't Tor's fault, but bad OPSEC on the part of the users.

Also, it is worth considering that debt collection agencies are very good at finding people, and very bad at upholding ethical standards. Going to prison is not the worst case scenario.

Is it actually profitable to do that sort of unethical activity though? These aren't exactly loan sharks right?

If you think you can't get caught because you use Tor, I know of a few people who can testify otherwise. See, e.g., Ross Ulbricht and Christopher Grief, to name a few.

Go to the public library and use a pc there? Or a free wifi in a mall?

See the previous point about Ross Ulbricht (arrested in a public library)