Most of the NeXT folks I worked with left many years ago. Perhaps generous stock options contributed to the exodus. Or perhaps once OS X was clicking along with a new Window Server and graphic sub-system, replete with an Objective-C-based Cocoa API, they felt like the work was complete.
There are nonetheless many of the old guard still there — presumably navigating the Swift-UI transition.
And Craig Federighi is, in my opinion, a very capable captain to be steering the OS ship for Apple.
Those that first put out Apple HIG, along with other software UX expert left as well. After Tim Cook fired Scott Forstall and named Jony as the CDO.