I think the present limitation has more to do with the fact that karma is dolled out anonymously, which means there's no way to back up the legitimacy of your earnings from the status of your upvoters. It doesn't matter if karma is infinite, because there are a finite number of socially approved participants. This is why national currencies have value in the first place: because valuable organizations/individuals use them to transfer wealth. Money is just a communications medium, after all. One stuck in a pre-web mentality. Existing web currency manipulation systems are as primitive as a YouTube that only played TV in your web browser.

If you make karma transferable and allow negative values (thus creating debt and preventing runaway inflation), you could offer a genuine relativistic currency, in that individuals could back up the worth of their unit independently, without relying on trust in someone else (e.g. the state apparatus).