Answer
The punishment of Shirk as narrated by the Quran is that Allah SWT will never forgive shirk as it is such a sin that has no merit for forgiveness.
When we study the Holy Quran thoroughly, we get to understand the merciful nature of Allah SWT. We get an idea from the Quran that even though Allah SWT will not forgive those who commit unpardonable sins, His immense blessing has the right to dissipate Hell altogether. Resultantly, when there is no place of punishment left, the punishment itself will not survive. This means that the criminals will be annihilated along with hell. When and if this happens, it will also be a clear display of mercy of Allah SWT.
When Allah SWT speaks about unpardonable sins, it is in terms of His law. Allah SWT does not have any personal issues as far as punishments are concerned. Allah SWT is “Al Ghani” meaning The One who is self-sufficient/The One without need of anything/ The One who is completely satisfied/ The One who is free from any wants and The One who is free from any dependence. Allah SWT is least bothered if we worship Him SWT or not, if we believe in Him SWT or not.
The actual problem is that this is the height of immorality and discourtesy on our part to not acknowledge someone who has given us everything. We are completely indebted to Allah SWT and His mercy. We owe each and everything from within us to the outside world to our Master. Our each breath, heartbeat, abilities, our strengths, our organs and body parts and our souls etc. are in debt to Allah SWT. In return to all these Mercies of our Creator SWT, how would we justify this kind of negligence and carelessness towards our Creator SWT. This is such a big injustice to the One whose uncountable blessings are enjoyed by each one of us every single day. People don’t take and consider this thing as a moral problem. They often speak about morality and rights of others. However, the foremost right of Allah SWT is to admit what is His. Whatever He SWT has given, should be associated to Him. It is an irony that we breathe in His air, eat, sleep and drink from His treasures and then totally forget Him by paying thanks to others. When the blessings are from Him SWT, the thankfulness should be to Him.
There are two places in the Holy Quran where Allah SWT has described the aforementioned things (presented below):
Surah Hud Ayah 107:
[They will be] abiding therein as long as the heavens and the earth endure, except what your Lord should will. Indeed, your Lord is an effector of what He intends.
Surah Al Anaam Ayah 128:
“He will say, “The Fire is your residence, wherein you will abide eternally, except for what Allah wills. Indeed, your Lord is Wise and Knowing.”