Basically, when every player gets older and experiences significant decline in performance, plus there are more young players who are great and have potential, those who are older will often sit on the bench.
Moreover, for Real Madrid, which already has quite lot of young players and can always be relied on when they are part of the main squad, this is also the reason why Ancelotti did not try to retain his senior players when he decided to leave and retire.
But by the way, it cannot be denied that those who are older and decide to leave in search of big salary, the last resort is Saudi football and there are already several players who have done this.
Everything is clear and Joselu also accepted the offer made by Al Gharafa and played in the Qatar League, but this will be the same because Saudi and Qatar are two countries that both develop football very well.
I feel like it is not just age, because Ancelotti told Kroos that he has a place in the team if he decides to come back on his word and not retire. I think it's more about talent and what you can provide for the team, Modric is older, and he still has one more year at Real Madrid, he is going to be something like 38, and still play there anyway.
Of course we have seen Modric be a lot more bench player than he used to be, so he is not going to play a ton of minutes, but at the end he is still there to provide something at the last 20-30 minutes of the games when they are having trouble. Aside from that, they have a very strong midfield and offensive line, so they do not need any players that would help them at that position any further.