Since Alba scored the 2-0 goal in the European Championship final, something very very very few fullbacks manage to do, and been the best LB in La Liga along with Marcelo, and was a big part in our treble winning team. Alaba has done nothing Alba hasn't done in top football in terms of achievements. Style of play is of course open to subjectivism, of course most Bayern fans like Alaba's style more, while Barca fans do the same with Alba. But achievements are not, and I don't see a legit reason why Alba vs Alaba isn't a close call at LB (you can add Marcelo in the mix as he's as good as both - different style though).
Pique is better than Boateng currently (best CB in the world last season), and Lahm is mostly a midfielder these days, and even if he would play RB, Alves has been the best RB in the world in at least 5 of the last 8 seasons, including the last one.
Sure, Lahm brings more safety in defence, in the sense that you feel more secure with him at RB instead of Alves, but Dani does several important things going forward that Lahm just can't do, and that comes from the fact that he is technically superior -- a better passer and a better dribbler.
As I said, I'll give you Neuer and Vidal. I'd understand why a Bayern fan would also have Lahm over Alves, and Boateng over Pique (although for me Pique is better and has been one of the best CBs in the world for the most part of the last 6 years) and Alaba over Alba. 6 players from both teams are non-negotiable though so to speak, and would walk in the other team with ease. Neuer, Busquets, Vidal, Messi, Neymar, Suarez. Neuer is obvious, Vidal is one of the best box-to-box players in the world, and Busquets is one of the best and most gifted players to ever play at DM. Then you have Messi, Neymar, Suarez where it's clear, although I suppose you could have Lewandowski at CF and still have a great team, even though Suarez is a better striker. Depending on the system you could maybe find a place for Muller. That's about it. But, and I speak as a Barca fan, for our first 11, I would only take Neuer, Vidal, and maybe Boateng if he can play LCB, next to Pique (alhtough I have a feeling that might not work at all).