So Douglas Costa is the alternative to Sanchez? Sanchez is a world class forward with a high work rate who could play anywhere in the attack. And would probably start most of the time over Aguero, Jesus, Sane or Sterling. Since he is either better or at least on the same level with all of them.
Signing Costa would be like signing an older back-up to Sane and Sterling. Who is just as tricky as those 2. But not as productive or good. At least in terms of goals and assists.
So it's either a back-up winger for the best front 3 in the league (IMO). Or we sign one of the best forwards in the world and have a front 3 ready to rival Barca's front 3.