This kid’s decision making looking more curious by the minute. Is he hoping to get playing time at Real Madrid this season? Honestly, he won't. It was a move to steal him from Barcelona. I feel a way about this decision to go with a team that promised him game time at 17 (An almost impossible task at Real Madrid), rather than one that gives him another year to develop with his current team. Anyways, there might be a myriad of factors that influenced his decision but the deal made it seem like immediate integration into the first team was a major factor; In which case, I just wish him the best.
I saw a few snippets of the press conference where Arda Güler was presented as a new player of Real Madrid and there he said he wants to become a legend of the club. I did not know that player before but he is definitely not lacking confidence lol. I think though that he will have a very hard time to have any playtime at all, as you already said.
Transfers like that where very young players (let's say younger than 20 years) directly join one of the absolute top clubs like Real Madrid, Manchester City or Bayern Munich have not worked out that well most of the time in the past. I think in general it is better to first go to a tier 2 club for a few years and then join the absolute best clubs.