Yeah, I may have been overstating the effect that the boiling water would have on the plastic. Like I said, it varies depending on the composition of the plastic (hell, some mattel figures are decaying right in the package:
https://www.poeghostal.com/2010/12/the-mystery-of-the-plastic-mist.html and the temp and duration it is in the water for. After all, it all comes down to the molecules, and even water, H2O, will react with the plastic, especially under heat.
Now, the decay rate will not really be noticeable, even 20 years from now. To the human eye, there will be no difference between a not water heated one and one that had been heated by water. But the actual molecular structure of the plastic may be a bit more weakend due to the heating by water. It's not like it's gonna turn to dust or anything.
Now this probably won't bother may people, but I'm REALLY picky about my figures and want them to be in very good condition, so I'll try to avoid anything that may have any possibility of damaging them.
And even if the water didn't possibly cause damage, the hair dryer method is just so much more easier and effective.
Really, sunlight is your worst enemy. I'd rather boil a figure than leave it exposed to direct sunlight for a long duration. And try to wear gloves when handling your best figures, the oils on your hands can cause damage as well.
But really in the end, you could take 2 1980's He-man figures side by side; where one was new in package and pristine. and one was opened and played with and the differences, and the plastic degradation wouldn't be that noticeable between the two.