Disagree. In Utero was released late 1993, whereas Cobain died early 1994...a relatively short period after. He also recorded the celebrated MTV Unplugged right at the end of 93.
I think In Utero is a better album than Nevermind IMO It's certainly more grown up and darker. As his death was in relatively close to In Utero'
s release & recording of MTV unplugged, in my mind thats close enough to still be regarded in his peak. He recorded the MTV Live n' loud special in december '93 and that was awesome!
In all honesty this whole 'peak thing' is largely subjective.
My point was had he lived like Amy Winehouse I strongly believe he would have been incapable of coming close to the work he had previously produced as he had arguably destroyed himself creatively like musicians Syd Barratt; Peter Green; Roky Erickson; Sly Stone; etc via a combination of chemical substance abuse & mental health problems.
Hence he was rapidly descending from his peak (like a mountaineer falling down a Himalayas mountain) when he died.