I mean I would understand either decision.
Inaki hasn't really done anything to be benched like this, but he's also a 25 year old small goalkeeper who's lucky to even be #2 at the club considering he has underwhelmed for years and couldn't grasp the one shot he had on a loan to make something off himself. Salary cap issues among other things is why the club haven't prioritized a more proven backup for ter Stegen or even legit competition.
Then you have the guy coming out of retirement who might or might not get back to his old level. If he doesn't he will cost us a game, but if he does he might be the one who saves us in a KO match, and he has gradually gotten better too.
It's not a perfect decision either way, but he's essentially betting on the guy who has proven he can be *it* instead of the guy who has proven nothing in his career apart from this recent good run which might be a false dawn for all he knows.