Still don't know what you guys are on about here, really. Frenkie and Cass have different skillset and strengths. The Coach sees this and uses that to fit into his gameplan depending on opponent.
I have been a great admirer of Frenkie's skills and talent but I would be mendacious to say there are no couple of things that Cass can be said to be better at, obviously, as CDM.
They are both good. Rather than bicker about who is better, I leave the coach to sort that out, expecting that either will turn up perfomance wise when called upon...... while I savour the feeling of at least a position that Barca can claim to have the luxury of quality options on the bench, either ways.