The key point was imo, the court case of Jonathon Majors, whose role as Kang was supposed to be upcoming villain which will then usher in DD, got caught as a time where all the lead up movies and disney shows are playing out.
As usual, even without any sort of court judgement at that point of time, Disney chose to draw and cut the line, which meant Kang's storyline now have to be rewritten as a one off Ant Man boss despite the credit scene hinting at a much larger storyline.
Since there's no Avenger's level villian now, they got DD in but have a headache about who to cast since he was never planned to appear so early. Lump that together with all the Guild Writer's strike and DEI nonsense resulting in poor sales, and also the ''lesser'' superheros that's now taking the grand stage, getting RDJ back as a shock factor was one expensive solution that they hope will work, esp with Chris Evans coming back as well.
