Indeed, Ralbag in his commentary there rejects the straightforward understanding of this miracle. For such a miracle to occur, nature would be changed in ways surpassing any of the miracles performed even by Moses whom no miracle-performer can actually surpass. In fact, for Hezekiah to even ask for such a miracle would be utterly foolish, which, based on context, is not an adjective we wish to apply to Hezekiah.
If I understand it correctly, Ralbag's interpretation goes something like this: The rays of the sun are sometimes refracted by the movements of the clouds. Where a ray should have hit point A it might end up hitting point B. Now the miracle was going to be that the clouds would move in such a way as to make the rays of the sun move 10 ma'alot (however you want to translate that) from where it was. Hezekiah saw that the natural conditions present, such as wind, mist, etc. were such that the clouds would have been moving in one direction anyway, so that Isaiah saying that the sun's rays would move in that direction would not be much of a miracle. Instead Hezekiah demanded that the natural movement be reversed, and the clouds move in the opposite direction from where they would have been moving based on the natural conditions. Thus, the miracle was that God spontaneously moved the clouds in the opposite of their natural direction to a point where they would refract the rays of the sun 10 ma'alot from their original position.
Thus, nothing actually happened to the Sun or Earth. Thus, it would not be a miracle that would surpass those of Moses, nor would it utterly destroy the world.
In Milchamot Hashem (Wars of the Lord) Book VI Part 2 Chapter 12 Ralbag discusses this again, saying the same basic idea:
If someone were to raise the question of the miracle of the shadow [moving backward] performed [by Isaiah] for Hezekiah — which might be thought as entailing a change in the order of the movements of the sun — we reply to him [as follows]. It is obvious that this miracle did not result from a change in the order of the motion of the sun. If it had, Scripture would not have attributed this [backward] movement to the shadow but rather to the sun; for the movement of the shadow is consequent upon the motion of the sun. Moreover, if this were true [i.e., that the sun has changed its motion], it would not have been possible for Hezekiah to say "It is a light thing for the shadow to decline [forward] ten degrees". For it is not possible for the order of the sun's motion to be upset with respect to quickness any more than it is possible for [its regular motion] to be upset by reversing its motion; the miracle is the same in both cases. The miraculous element in this event is as follows. In a receiving [body] of some thickness, which operates like a mirror, the [reflection] of a heavenly body can be seen. This is the explanation of haloes and the rainbow, as has been demonstrated in the Meteorologica. Now when a cloud is under the sun, the sun is reflected in it. If this cloud moves quickly, it can move the ray of the sun along with it and cause the ray to be seen not in its proper place; for the cloud will have moved for a while before this impression disappears from it. It is in this way that when a cloud passes under the sun you see the ray of the sun not in its place. This is the explanation of what happens with flowing water: a man sees his reflection duplicated. Because of its quick motion in the water, in which the image is reflected, flows by and the image disappears from it only after it is reflected in another part of the water. [Now,] since Hezekiah had noticed the movement of the cloud in a certain direction, he said that it would be easy for the shadow to incline 10° in the direction of the motion of the cloud, because of the speed of that motion. But it would have been a miracle if the shadow had gone backward 10° in an opposite direction to that of the motion of the cloud. Thus, the miracle at that time consisted in the reversal of the cloud's motion from the direction it was going at the moment, as fixed by the prophet. It is evident therefore that the miracle did not occur in the domain of everlasting phenomena [i.e., the heavenly domain].
(Feldman translation)
