There are several considerations when saying Kepa and De Gea in this case. First in terms of salary this will waste a lot for De Gea on the other hand, in terms of age is also one of the reasons considered and the most important is the contract. When they borrow Kepa then it is possible that this will only be one season as originally planned in contrast to De Gea whose status is a free agent because there is no way they will do it with just 1 season so this is also one of the reasons that makes sense why Madrid did not do it with De Gea because it would have been a bigger waste of money. Even though they will not have a problem with it, they also realize that business is the most important as long as they can minimize expenses then why look for something a little complicated to do.
The most important thing is that Kepa justifies the wishes that are placed on him by Real Madrid. To be honest, the conditions, and especially the amount of rent, are very strange and unusual. I understand perfectly well the positions that Thibaut Courtois has earned at the club, that it makes little sense to take on this position of an equally strong goalkeeper for a short period of time. Another question is how this will affect the game of Kepa himself, who knows perfectly well that he is a temporary player at the club, and Chelsea don't really count on him after the loan.
Kepa realizes that he at Chelsea will sooner or later be wasted if he does not improve himself and if indeed his performance in Madrid is satisfactory it does not rule out the possibility that there is an option for this player to be glimpsed by Pochettino because we know how he was at Chelsea which was almost invisible because of Mendy then returned to the main squad before he was currently on loan.
Being in Madrid with the condition of the main goalkeeper must be maximized well by Kepa and my hope is that he has a good performance unlike at Chelsea where sometimes he makes blunders for his own club.