Looking at ucl 2015 final:
Ter stegen
Alves (now sergi) pique mascherano (now lenglet) alba
Rakitic Busquets Iniesta (now Vidal)
Neymar (now coutinho) suarez messi
By now is the 2nd leg anfield starters, of course.
You could argue about different opponent. One can say current liverpool is even stronger than 2015 juve. I say if it's only one match at neutral stadium, the match would lean more towards 3-0 result than 0-4. Why? Our away mentality.
Second, age. There are only 4 changes in the formation within this 4 years gap. Coutinho and Neymar are about on the same age, even. The rest of the players are the same player,hence the same quality, yet 4 years older. One could argue some players may be better today than 2015, but you can say it in opposite to several others.
Still it cannot explain and it will never be able to explain how such anticlimatic ever exist in football. How 4-1 could be 0-3, how 3-0 could ve 0-4. How weak our mentality at away games. Did we play xavi from the beginning during semifinals against bayern? I have to check it again. Because like the other man said, he was the last captain material barca had.