Researchers know that almost all this heat is generated by the decay of radioactive elements such as potassium-40, thorium-232 and uranium-238
Half-life of (K40, U238, Th232) is (1.2, 4.5, 14.0) x 10^9 years. Age of Earth is 4.5 x 10^9 years. That explains why we still have such elements...
Did the Malaysian government just make a statement to the families based on a statistical probability?
Maybe based on the families + China exasperation. The debris were not found, let alone the plane, and families kept (desperate) hope. Malaysia could have made a similar announcement earlier.
The rule on staying alive as a program manager is to give 'em a number or give 'em a date, but never give 'em both at once.