See this bolded part is where I really disagree. I actually don't think he has been stubborn enough to this point. He has compromised on his style far more than I expected and for one I would prefer him to take more of "this is what we are doing, win lose or draw" approach. But last season he had to compromise to a degree, this season less so.
In terms of Onana coming good, I don't think it will take that long. Let's see where we are at when the players critical to our tactical set up like Martinez (proper Martinez, not the half fit version we saw this season), Antony and in particular Shaw are back in the side and get a proper run of a few games together. It was interesting that the players went to him after the game on Saturday, he clearly retains their faith and is popular with them which is half the battle sometimes.
The decision to get rid of De Gea was absolutely the correct one. Even if Onana turns out to get worse and worse, it was still the correct decision to move on a goalkeeper that was dreadful at most areas of goalkeeping and wasn't even that good anymore at the one area he used to excel, while showing very limited ability or willingness to adapt his game towards the managers stylistic preferences. If Onana wasn't available this summer we'd have still moved De Gea because of how deep we had to play to compensate for the goalkeeper last season (we spent the majority of the season with a similar defensive line height to the likes of Forest, Bournemouth and Everton), it was not possible to play front foot possession based football, or even maintain an effective high press, when the defence has to sit so deep. That's why the De Gea v Onana debates are ridiculous, I understand why they come about because it's obviously comparing players in the same positions but the reality is it's about the impact on what those in front of them are able to do; Onana may or may not be the right goalkeeper for how the manager wants us to be playing in 12-18 months, but we all know De Gea definitely wasn't the right goalkeeper for that. Ten Hag tried playing his way with De Gea last season, and we saw very quickly he couldn't do it, so there were compromises made to get us through the year, but Ten Hag would have known that the change needed to be made very early on (hence us going for Trapp and Sommer late in the window after Brighton and Brentford).
Giving De Gea another year would have meant another year of compromise. I guess the irony is that we got rid of him and still had to compromise due to the injury list, which has led to the line dropping deeper again, but that will change as players come back into the team.