Sure, if you take "destroy" Shutterstock hasn't destroyed iS and TS hasn't destroyed either of them. It's safe to presume they poach some sales from each other, though.
Actually I'm seriously starting to wonder if all the promotion of TS to 'selected IS customers' has in fact been a gift to SS?
They might have enlightened such customers about the advantages of a subscription but that didn't necessarily mean that they went on to buy from TS. If a potential customer bothers to do any research whatsoever on subscription packages they are very unlikely to choose TS.
My stat's for this month project Istock to be 37% down and SS to be 35% up (and massive BME) compared to March 2010. It's difficult to believe that the contrasting fortunes of each agency are not somehow linked by a common factor.