Failover in this scenario would require instant, 100% replication of a near-zero latency system.

I'm no expert, but I'm fairly sure having trades 'disappear' or any other loss of consistency would be worse than downtime in this case.

Unfortunately sometimes fail over isn't a solution. Until they've at least narrowed down the problem, a fail over system may make things worse.

Personally, I'm curious about the testing that was done - given the complaints and problems that have been reported so far, did they even try to run the new and old systems simultaneously for any period of time to verify that the various components could all keep up properly? Or at the very least, did they try to test the system at full load for a decent period of time?