First, you have to consider that Luan is probably thinking going to Leipzig is a better move at this point of his career than playing for Barça B and then going on loan because there is no way he would get first team minutes with Alba still playing on top level. Can't really disagree with him if that's the case.
Second, Alba is much better than anyone we have on the right. We need to find someone who can replace him for 10-15 games for the next season or two (veterans like Filipe Luis or Monreal) and then we'll probably buy someone who could replace him longterm. I think Barça still believes Miranda could be a starter in the future. I'm not as optimistic but we'll see how he'll develop.
On the other side, while both Semedo and Roberto are good players none of them should be seen as unquestionable starter (like Alba) and it's ok we're looking for upgrades. There are not many realistically available quality RBs out there though so going for diamond in the rough with cheap players like Wague and Emerson makes more sense than spending 50-60M on someone who might not be much of an improvement (if at all) over what we currently have.
That said, it's very likely we'll still bring a young LB for Barça B with rumours Miranda will leave on loan to La Liga team (I'm not sure this is a good idea but we'll see). There were rumours about another Brazilian named
Riquelme a few weeks ago so it's possible we're considering him. He'll only turn 17 in summer so he could start playing for Juvenil A if we'd get him. I'm sure there are many other players on our radar too so missing on Luan won't be the end of the world.