A player alone cannot take a team to winning trophies, it takes every member of the club including those who are also off the pitch, it's a combined effort if not Harry Kane would have won as much trophies. Yamal is a great player, having saw his relevance at the Euro then unarguably Barcelona could make good use of him even better to maximize their chances but what will be his effect against Madrid or Manchester City likely in the Champions League.
The goalkeeper and defenders are the two combination that helps most of the team to win the league no matter how quick the forward are to score goal for the team. This is why Harry Kane doesn't seem to have Trophy corresponding to the number of goals he has won in the Spur and in the international matches and he is already getting closer to his age where soon he is going to retire but why all of a sudden we think Yamal has won the World cup, even Mbape that won the World cup in 2018 has not been able to win the Champions League in 2024, this comparison should rest.
Modric is still there to do what Kroos does but it won't be effective because the physical strength is no longer there and he would not be given as much playing time. Valverde will be the centre of attention, let's see what his performance will be.
I'm not sure Modric will last more than a season in Real Madrid, it could be that Real Madrid doesn't want to see exit of two legendary players. If they win another Champions League which is likely because of the presence of Mbape, he will have another trophy in the bag and will break another record but personally, I don't see much of his presence in the team with the good players Real Madrid has right now.