Re Capricorn23's comments...
It seems (or as we say in investigation speak, "it is likely") that if the intent was to end the flight in a place where it would be very difficult to find, then whoever flew the aircraft apparently considered the optimum route to achieve that end. Which could be one reason that the track after rounding Sumatra didn't (apparently) get closer than 1200 miles or 2000 kilometres to Diego Garcia. The particular track chosen after Sumatra (as per Inmarsat data) appears to be close to an ideal track to end in an isolated area of vast deep ocean, whilst always remaining more than 1000 miles from any land.