Props to @Serghei for the pictures, long analysis and hard work to show his argument through them.
And of course Serghei is absolutely right. It's ABC of modern football that you cannot defend with 8 men, and if the 10 players do not defend cohesively as a unit, the defensive plan will fail no matter what. Even teams that play 4-5-1 or similar formations and leave a man upfront in the defensive phase as a counter-attacking outlet, this man is never a walking or non-walking statue, but presses constantly either the CB or the DM.
Additionally, about playing it 'conservatively' that's exactly what we saw from Ernie, leaving the defensive deadwood upfront and defending with two banks of 4 in a low block. Again, insufficient in modern football. It makes a team like mediocre Roma 17/18 look like an offensive machine. Besides, the 8 that defend do not have individual qualities that would make them good players in that kind of system.
Ernie wanted to play Pep's high continuous pressing, and saw already in the first month that it's not possible with Messi and Suarez upfront. He masked it in the best ways he could, but still the outcome was tragic. His lack of bravery and personality meant the devastating regression of the last 3 years.
Setien had great ideas like his initial 3-4-3 that was a brilliant offensive plan, but soon realized that he had players unwilling to follow instructions and learn something new. Let alone that he didn't have again bravery and courage to bench the deadwood. It's interesting to note that best period under Setien came with Suarez injured, and when he was sloted back in the starting XI after the break, things were going crappier with every new game.
We desperately need someone to bench (if not kick out of the club) the deadwood, or else no renovation will ever happen.
Let's hope, only hope Koeman will be this man and Barcelona can defend again with 10 players
PS: By the way, I agree with Serghei that Messi being in the starting XI means we are out of any CL knock-out game.
And also to Messi apologists: even his offensive game is not the same since the end of 18/19. He has became very predictable there.