An unbeaten run that would mean nothing.
It does means something. Just check out how hyped and revered Arsenal's Invincibles are, in the PL. Its even tougher to achieve than winning CL B2B. While you can afford one or two losses in CL, one loss and kiss good bye to unbeaten season. Efforts do mean something, even if you aren't fighting for the so called holy grail aka CL.
But, Yeah, I support your point of giving youngsters chance, atleast Dembele in classico & maybe alena in future games. Starting Iniesta won't do any good, he is going to leave anyway. Demebele isn't an impact sub, so I don't think its a good choice to bring him for Iniesta. He needs to get adapted to classicos, because although this one is irrelevant, the future ones will have a lot at stake. And, this one is the best for a youngster, because you have nothing to loose. So, Dembele can play freely without any pressure.