I worked on Finder/TimeMachine/Spotlight/iOS at Apple from 2000-2007. I worked closely with Bas Ording, Stephen Lemay, Marcel van Os, Imran Chaudry, Don Lindsey and Greg Christie. I have no experience with any of the designers who arrived in the post-Steve era. During my time, Jony Ive didn't figure prominently in the UI design, although echoes of his industrial design appeared in various ways in the graphic design of the widgets. Kevin Tiene and Scott Forstall had more influence for better or worse, extreme skeumorphism for example.
The UX group would present work to Steve J. every Thursday and Steve quickly passed judgement often harshly and without a lot of feedback, leading to even longer meetings afterward to try and determine course corrections. Steve J. and Bas were on the same wavelength and a lot of what Bas would show had been worked on directly with Steve before hand. Other things would be presented for the first time, and Steve could be pretty harsh. Don, Greg, Scott, Kevin would push back and get abused, but they took the abuse and could make in-roads.
Here is my snapshot of Stephen from the time. He presented the UI ideas for the intial tabbed window interface in Safari. He had multiple design ideas and Steve dismissed them quickly and harshly. Me recollection was that Steve said something like No, next, worse, next, even worse, next, no. Why don't you come back next week with something better. Stephen didn't push back, say much, just went ok and that was that. I think Greg was the team manager at the time and pushed Steve for more input and maybe got some. This was my general observation of how Stephen was over 20 years ago.
I am skeptical and doubtful about Stephen's ability to make a change unless he is facilitated greatly by someone else or has somehow changed drastically. The fact that he has been on the team while the general opinion of Apple UX quality has degraded to the current point of the Tahoe disaster is telling. Several team members paid dearly in emotional abuse under Steve and decided to leave rather than deal with the environment post Steve's death. Stephen is a SJ-era original and should have been able to push hard against what many of us perceive as very poor decisons. He either agreed with those decisions, or did not, and choose to go with the flow and enjoy the benefits of working at Apple. This is fine I guess. Many people are just fine going with the flow and not rocking the boat. It may be even easier when you have Apple-level comp and benefits.
My opinon; unless Stephen gets a very strong push from other forces, I don't see that he has the will or fortitude to make the changes that he himself has approved in one way or another. Who will push him? Tim Cook, Craig Federighi, Eddy Cue, Phil Schiller? The perceived mess of Tahoe happened on the watch of all of these Apple leaders.