Perry is the one who came in and said the plan is to Rebuild with the draft and youth. So his record is expected. There are even people calling to tank so you can't blame him for the record. A rebuild is supposed to take a few years (of being bad.)
An important part of a rebuild is sticking with the plan. It was Mills who wanted to keep Morris and Perry held him off to stick with the plan to rebuild.

There is also a factor that isn't being mentioned. World Wide Wes. Perry is a longtime associate of Wes. https://www.gq.com/story/william-wesley-worldwide-wes-nba-basketball And Wes is the behind the scene Factor that will try to funnel talent to the Knicks (in the same way that he gets talent to Coach Calipari at Kentucky.) He's like the quiet consigliere of Leon. I'm sure Wes advised Leon to keep Perry. If Wes is silently, unofficially associated with the Knicks FO then he'd want a GM thats familiar and trusted.