You're exaggerating his struggles last season. The only guard that played better defense on our roster last year was Okogie. Booker went from being very good to very average... but a lot of that was him being tasked with guarding the oppositions best guard (again, unless Okogie happened to be on the court). He also was doing far far more ball handling than had been asked of him before.
Furthermore, Ayton was about 10 times better than Nurkic at dealing with penetration and cutting guys off before they get to the rim. Once one of our guys got best last season it was either a layup or we had to collapse the defense, leading to wide open threes. Ayton's ability to defend all the way out to the perimiter gave our guards more leway to play aggressive on D. Nurk has no ability to defend away from the hoop so our guys have to sag off and try to keep dudes from getting by them.
This isn't not a denial of Ayton being an inconsistent, gutless, headcase... but he did do somethings very well, and that was one of them.