it really depends on how many 1st team games EV is planning to play him if he is promoted this season. if EV plays him for 10 games in La liga and Copa del rey together then I would promote him. If not then play him in Barca B. I think 10 1st team games are more useful than all Barca B games in the third division.
You can have both, and this is what the coaching staff apparently seeks to do, too. Give him regular playing team at Barça B and give him the games in which Alba rests. He wouldn't be needed on the bench during most of the games Alba starts anyway, so why would you rather have him on sitting on the cozy seats at Camp Nou instead of playing for the B team?
I am very happy that we're not buying another back-up but instead rely on our own players. Would have been a terrible sign to not have Cucu or Miranda be the primary back-up for Alba. Yet I can't say I'm too happy with the choice here as I'd have gone with Cucu. Not because he's necessarily more talented but because he's a) more experienced and b) we could have tested both players – Cucu as the primary Alba-BU, Miranda at Barça B. Next year, if Miranda proves to be better you could still have promoted him and let Cucu go.
Now, however, we maneuvered ourselves into a position in which Cucu will most definitely leave the team. Being the older and more experienced player, the promotion of Miranda is a clear sign that the coaches don't see much in him. I would have preferred another year of both at the club to see their development. Only if they really don't trust Cucu I understand that decision – otherwise they should have promoted him even if Miranda is considered more talented overall. It's just not that easy to foresee the development of players at such a young age.
But it's a compromise, I guess. Valverde wants an external replacement at LB (can't fault him for that from a coach's perspective) but our board insists on making use of our youth. That's definitely the right call from our board, this is a prime example of a situation in which we HAVE to give La Masia a chance or else we might just as well shut it down and save the money. It's understandable, though, that the board does not dictate Valverde whom to trust so giving the green light for Miranda is an acceptable compromise even though it's strategically suboptimal imo.