The World Cup thing remains a theory from folks who never saw the medical reports of his. Just like many sided with the club later about him not doing an operation, and same with Fati. There is a lot of question marks about our medical staff in that period.
I doubt he has any regrets, he got paid fairly well, won almost everything to be won other than CL (he wasn't going to win it with us anyway). The World Cup is the hallmark achievement of any player and he was integral part of the winning side in 2018
He retired because his Barca payments finally run out?
I think it ends this season.
He renewed until 2026, spreading his wage of the last 18 months over 4.5 years. With a termination option that likely allowed to get paid while looking after his career elsewhere.