1)
What was good about the prophecy?
1.
Rashi: There will be Shalom in my days.
2.
Malbim (Yeshayah 37:8) #1: Chizkiyah realized that he did foolishly. Hashem's word is the proper consequence of my foolishness, or the punishment for my pride.
3.
Malbim (Yeshayah 38:8) #2: There are two aspects of the prophecy. (a) It will be bad for my descendants. This can change if they will improve their ways. (b) It is inferred 1 that evil will not come in Chizkiyah's days. This can never change.
1
What is the inference? Yeshayah did not say that there will be Shalom. If they do other sins, there can be war! Perhaps Malbim means that for this Aveirah, the punishment will not be in Chizkiyah's days. (PF)