The engines hours were irrelevant as RR insisted VTTS operated their engines up to a maximum number of cycles. A cycle being going from throttle fully closed to fully open and back again - or incrementes thereof.
In the early years VTTS were using engine cycles at an unsustainable rate due to the sort of flying they were doing, but adapted how they flew transit flights and even displays to essentially try and leave the throttles alone and in the same position as much as possible. To this end they were managing to get a lot more "hours" from the engines and had sufficient cycles remaining for a few more seasons beyond 2015.
The engines destroyed by the silica gell were reportedly very nearly at their end of their allowed cycles anyway. Even if they were not it turned out this error wasn't to cause the end of flying. Of course this is just the official line VTTS have reported.