However, is the pressure 4000 metres below sea level well in excess of the crushing weight of the world trade centre towers? Also, there are many instances where black boxes have been well able to withstand excess heat for many days.
Ballpark figures, but a flight recorder typically is designed to withstand intense fire (1,000C) for about 30min and lower-temperature (250C) for about 10 hours. Plus pressure immersion to twice the depth of AF447.
There are few (if any) cases of aircraft fires burning anywhere near those durations, making the intense WTC fire something of an exception.