All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
We do not know why you think that it is impossible for the sky to fall on the earth while we see that some meteorites fall on it, and see, from time to time, the approach of comets –some of which are large – and if they were to fall on the earth, they would cause much harm to its people.
What is meant by preventing the fall of the sky on the earth is preventing its celestial bodies and planets from falling on the earth. Allah prevents them –out of Mercy to His Slaves – from falling on the earth, and if Allah had willed, then He would have done so.
Allah Says (what means): {Then, do they not look at what is before them and what is behind them of the heaven and earth? If We should will, We could cause the earth to swallow them or [could] let fall upon them fragments from the sky. Indeed in that is a sign for every servant turning back [to Allah].} [Quran 34:9]
Ibn ‘Aasshoor said in his Tafseer (At-Tahreer wat-Tanweer): “The term {…or [could] let fall upon them fragments from the sky} means, a piece of the celestial bodies.” [End of quote]
Allah knows best.