And it looks like her last hurrah since it appears she will probably have to alter her song styles to suit her age
Well now we definitely know it was a fantastic 'hurrah', whether it will be the last, only time will tell.
As to 'altering styles', every artist should do that, both because they themselves are changing - age being one of the factors and because the market tastes are also constantly changing.
Yet the alteration should not be so drastic in case of established acts like 'IU', that it discomfits those who are comfortable with the artist's perceived style. And Lilac seems to have achieved that balance reasonably well.
If they don't and keep reproducing the same style again and again, they won't last. And 'lasting' is what IU has been doing perfectly for a decade now.
I do hope she produces 'mature' music, it will be irritating to see her sing Boo or Marshmallow kind of songs and she knows that, has retired them.
The Overton window of Kpop itself might slowly move to older ages, acts like BTS, IU, Brave girls all will cause that. If it was teenagers who drove Kpop in 1990s-2000s - they are now in their 30s and 40s and IU anyway already has built her audience far broader than the teens.
Like fashion, musical tastes are often cyclic, you have already noted the rise of trottists, that is a moving away from the visual dazzle business of K-pop, and among K-pops IU's genre is far close to trot than others, so if at all she might adapt better to the cyclic shift.
It all depends on what you define as "K-pop", after all "pop" stands for "popular", so anything popular which can't be distinctly categorized as a genre, is pop.
So Whyknock should not retire, but continue the threads with historical analogies. Though I don't share your sentiments regarding IU, I thoroughly enjoy the posts.
And I hope IU as well continues to 'inspire;' you.