The sad part is Bellingham can become a bench player next season.
Look at Tchouameni. He was bought for 80 million Euros and he has been on the bench for Madrid and is even planning on leaving during this transfer window. There are tones of young players that have been highly rated like that but didn't have anything to show for it. A few even won the best young player of the year award.
Monaco even had a balon d'or clause to Martial's deal when they sold him to Manchester United because he was that good. So predicting that a 19 year old player going to Madrid would be among the best midfielders in the world for the next 10+ years is reaching. Very few players go to Madrid at such a young age and turn out to be the best in their position.
Now it depends on performance whether he can show what Madrid expects or not because if he doesn't then it will be very difficult for his future.
But on the other hand I still feel that in this case Madrid will try to give him hours to adapt to be better, the reason is simple, Kroos and Modric are getting older and indeed now is the right thing to give some young players a chance with their current condition.
I agree with this because only a good performance can save themselves from Madrid because when it is not then it is very difficult especially since Madrid can also still be said to be one of the clubs that is quite assertive especially with poor performance.
Many examples of those who fail to perform well are always reserves even for a long time and in recent seasons of course Mariano and Hazard are one example.
On the other hand, it is possible that this season there must be rejuvenation for the Madrid squad because as you say the age of Kroos and Modric is no longer young now and of course looking at the current conditions with Tchouameni, Jude and Valverde it can still be for midfield. as for the wing line there are already Vini and Rodrygo there is only a matter of seeing the attacker or targetman this time who will be the choice.