Ah yeah, that's a good point, I totally forgot about them, thanks!

I think the big difference here is that obviously decentralization is nice, but the real issue is that Linden could do it because they simply didn't become big enough to be a target. There were some attacks at the edges, but if you wanted to do what Linden did at Visa-scale, they would quickly put a stop to it with chargebacks.

Adversarial problems are far easier at smaller scale, but it is a good point, thanks for the correction! It may be more accurate to say nobody's done it where it could be conceptually feasible at high-scale/volume with dedicated adversaries (many of the Linden exchanges solved this with high exchange rates to buy in, e.g. Virwox).