I don't think a Messi that pressed or played much better would have changed the outcome in most of those CL losses. Might have been closer losses though.
You had a goalkeeper that rarely made any big saves in big moments, which is still true years later.
A defence that couldn't cope with pressure and was slow.
A midfield that was slow, not very athletic and couldn't withstand the opponent pressing up high. Couldn't break the press. If anything guys like Rakitic made it worse like with that backpass to Alba at Anfield.
It's nearly impossible to win with so many serious flaws.
I will answer both of your answers here, I admit that in 2019, Messi was a ONE MAN ARMY, but the matches in which he really played extremely well and the team to say that it was too weak, it's too far from me honestly. The match with Real Betis is an exception from that season, with all my respect for you. In the same way, I could have given the match against Real Madrid, exactly from the same season, and said that "look how well Barcelona played without Messi, and in an elclasico".
And if it is still about the Barcelona/Rakitic midfielders, in 2019, Rakitic really had a nice impact on Barcelona, if you remember the goal against Tottenham, from the same season, or the assists for Messi, in the UCL groups. (I don't know for sure if they were UCL groups or LaLiga, not to confuse).
As it comes to my topic and what you wrote, in 2019, you know very well that Barcelona had great opportunities to score against Liverpool at Anfield, and they all came as passes from Messi, but still, I can blame on Jordi Alba for missing that opportunity vs Allison? I don't think that Jordi Alba is paid to have a good finish in front of the goal, the same with Coutinho, who was good as an attacking midfielder, but Valverde always played him on the left side in attack, I could never understand why. And you also had Suarez, to score, since Messi created the chances and did not appear in the square area at all, but any phase when Suarez could score on goal, he always had three defenders next to him, which forced him to shoot from very weak positions.
I agree with what the Barcelona fans say, like the defense was extremely weak, Pique with his mistakes, Ter stegen with a lot of balls to save (2 goals at Anfield, mainly because of him), as well as Jordi Alba, but NO THE ATTACK wasn't brilliant at all, in all the famous eliminations from 2016-21, (2-0 atletico, 4-0 PSG/ 3-0 Juve, 3-0 Roma, 4-0 Liverpool, 8-2 Bayern, 4-1 PSG), you conceded a lot of goals and only lost 0-0, it's not like you lost a match 5-3, you scored 3 goals but the defense being very weak, you couldn't qualify further and from here my obsession with this problem, because you still had Suarez and Neymar and the goat in the team.