The arrival of Mbappe and Endrick has great potential to make Real Madrid even stronger but on the other hand, this will also make the battle for places in the first team even fiercer (particularly on the front lines). If Arda Guller ends up staying with Real Madrid next season, then he will also be in that fight. Obviously it won't be easy, because he has to compete for a place against Vini, Rodrygo, Bellingham, Mbappe, and Endrick.
This condition might also be a headache for Carlo Ancelotti because it's not just about determining the best decision but he must be truly transparent so that the decision does not give rise to assumptions among the players that might actually trigger jealousy and unhealthy rivalries.
I don't think Arda Guler needs to fight with these players for his position because Arda Guler is a midfielder and not a striker. So it really doesn't make sense if he competes with these players. In addition, Arda Guler must be able to compete with other midfielders such as Bellingham, Camavinga, Modric or Brahim. However, of the four names, only Brahim could take over his position.
Arda Guler doesn't need to compete for the first team because there are no longer any players he can replace, but if he becomes a more proportional backup like Brahim, he might still be able to fight. On the other hand, if a midfielder is injured, it will also give him a bigger chance to play. So this is just a matter of rotation so to fight for the main place I think it's still quite a while Arda Guler can do it.
Apparently, as you say, the players in the midfield that Arda will compete with are very experienced and proven players.
Arda Güler will prove himself with the contribution he makes and the selfless game he plays when he plays the game in the event of an injury or similar situation, and I believe that he will do this in the near future, because I think he is such a good player. This kid will continue to be Real Madrid in his career and this will not happen by sitting on the bench.