We See In Part

[38:15] What shall I say? he hath both spoken unto me, and himself hath done it: I shall go softly all my years in the bitterness of my soul.
This is a good promise Hezekiah makes, and one often on the lips of the person G-d has spared. But in the end, it was only a good promise if Hezekiah made it good.
Hezekiah is to be admired for his accurate self-knowledge, and his honesty. He admits that it was not for G-d's glory or honor, or even for the glory or honor of his kingdom that he was troubled over his impending death and that he wanted his life spared. It was for his own peace.
What did Hezekiah do with these added 15 years? One thing he did was father a son who would succeed him on the throne of Judah. Of the next king of Judah, Manasseh, the son of Hezekiah, it is written that he was 12 years old when he became king (2 Kings 21:1). This means he must have been born in the last 15 years of Hezekiah's life. Sadly, fathering Manasseh was not a worthy achievement. It was written of him, and he did evil in the sight of the L-rd, according to the abominations of the nations whom G-d had cast out before the children of Israel (2 Kings 21:2). In fact, G-d specifically targeted Judah for judgment because of the terrible sins of Manasseh (2 Kings 21:10-15).
This healing marked a sad turning point in Hezekiah's life. Up to this time, his faithfulness to the Father was total. After this, he became proud and selfish. Diplomats from Babylon brought from their king a message of congratulations on Hezekiah's recovery from sickness. Their intention was apparently to find out how Hezekiah's G-d had defeated Assyria, for they too wanted to defeat Assyria. Passing up this God-given opportunity to publish to the up-and-coming world power his reforms and revivals, his prayers and the L-rd's answers, Hezekiah instead showed them his armories and treasuries. By doing this, he demonstrated that his trust in
G-d had deteriorated, and he now trusted his military and economic strength. The prophet Isaiah, foreseeing Babylon's rise, reprimanded Hezekiah's lack of faith, telling him that in generations to come, Babylon would abuse Hezekiah's descendents
— and that Hezekiah, by his faithless influence on the diplomats, helped spur their evil cause. But Hezekiah, having grown selfish, took this as a good sign that during his own lifetime, peace and prosperity would continue — and he didn't care about future generations.
In this, we may see that G-d had a better plan than Hezekiah did in calling him home at the earlier time. G-d knew that if Hezekiah lived, he would give birth to this wicked successor. Sometimes it is best to simply leave our lot with the Father, and leave what even seems to be clearly good up to His wisdom.
In the chief end we should aim at, in desiring life and health, is that we may glorify G-d, and do good, and improve ourselves in knowledge, and grace, and meekness for heaven.