It could still happen, if Àrda Guler chances of getting more game time would be reduce ( Which will be reduced ), he'll have no other option but to accept the move to Bayern Leverkusen. Working with Xabi Alonso would be great and I believe in two to three years ahead, the both of them will be working again together for Real Madrid, so it's a win win situation for them.
Considering the arrival of Endrick and Kylian Mbappe, with the likes of Vinicius Junior, Rodrygo, Brahim Diaz and Jude Bellingham, I don't know if they'll be able to make up chances for him to play but it's up to Àrda Guler and Carlo Ancellotti, they'll need to have a man to man discussion prior to the new season to avoid disappoinment
Xabi Alonso needs depth ahead of his first Champions League Competition campaign with Bayern Leverkusen. I'm happy for him and hopefully, he'll finish strongly above everyone on top of the group stage in the Champions League Competition and getting to the Semi finals if possible. He has what it takes to do that.
While I do agree that it would be a possibility for some other team, when was the last time Real ever loaned out a player that they plan on using? I do not remember that at all, they like to keep that player training with them more than anything. Their training is top level and they build players to be physically much stronger, and much better so they want players to get their own training, that is the important part.
I agree that if it was possible for Arda to train with Real while playing for Milan was a possibility, then they would have done that without question, but even if they loan him to Athletico Madrid, which is literally right next door, it just means that he would train with them anyway, not with Real. So that is why they will keep him, maybe 100 minutes all season long, maybe 1000 minutes, maybe more, but they will keep him.
It is not about getting playing time, and considering how good they are, I am sure they will guarantee the title early again, which means that they will end up with putting him again, just like last year. That is what he should wait, keep on improving, keep on training ,and be better at the end of the year when his time comes, will make things a lot easier for him.