According to Zak, Stella was one of his preferences for TP since back in 2019, however Stella refused the role back then because he did not feel ready. That's why he pursued Seidl. Surely his influence on the team must have been quite obvious to see for a CEO to notice it just one year after joining. Stella has been with this team since Fernando brought him from Ferrari. He was there during the worst time for McLaren and he was one of the few who saw the downfall of the team transpire. I would argue the downfall started a couple of years before Stella joined, but still, 2014/2015 is widely regarded as the beginning of the downward spiral for McLaren.
He is an exceptional talent with amazing problem solving skills. I think most people don't know he has a phd in mechanical engineering. A true engineer with a critical mind who keeps an eye on every little detail of the team. I am sure Stella noticed things going slightly south between 2021/2022, but because he was not the TP, the only thing he could do at the time was suggest changes.
I think it's not fair to judge Andreas Seidl for the short tenure he had at McLaren, but I am 90% confident that hiring James Key was one of the few wrong moves Zak Brown did on his journey to reviving this team. Cannot blame him as he was highly rated in the paddock a couple of years back, but he had hardly achieved anything worthy of a "wow" back then. In 2021, he just took over the mature concept developed by Pat Fry in 2019 (the 2019 car saw McLaren jump from back of the midfield to the front of the midfield). There were bad signs already in 2021 because the in-season development was laughable. Got completely slapped by Ferrari and went from fighting with them for occasional podiums at the beginning of the season to barely scrapping for points at some rounds towards the end.
And then 2022 was the real test for him and he failed. He came up with a bland looking car and admitted to taking the "safe cautious way". I just consider that fancy words for "this is the best I could do". Zak mentioned it somewhere that early 2022 was a tipping point for him because there was no sense of urgency in the technical team for the lack of performance. An upgrade at France was promised to deliver a big step, and when that failed the targets the reaction to the failure must have been pretty mild.
I am glad Zak took action. Key must have been overriding more talented people since he was the technical director. Once Stella came along, Peter Prodromou probably was given the reins in aerodynamics and boy did he deliver. Let's see what Key will do at Audi for 2026, but I have a feeling he will fumble that up too ...
The team is at a very good place right now, and it's almost criminal that poor management left some highly talented people on the sidelines. Of course the team has had reinforcements lately in the form of highly-rated hirings, but the team of people that initiated the turnaround last year were people that already were working at McLaren.
It just took a better leader to get things going.