It is all connected.
When you have slow CBs, teams will take advantage of it and use speed to force you to alter plans.
When you lack speed upfront (and I am talking real speed, not Trincao/Griezmann speed) teams can play highline because you can't take advantage of it.
With both, even the greatest passer will have difficult to connect to each other. Regardless of who is your midfielders/ attackers.
You need some real physical tools to force your game plan
True, it's all connected. That's why you have to replace old slow players with young and fast ones.
Why don't we wait to see Trincao play first before we pass judgment on him. And Griezmann is not a slow player. He may not have Mbappe pace, but he doesn't look and move like a slow player at all.
Messi, Suarez, Busquets, Rakitic, Vidal. These are all slow and old players, some are also lazy on top of that (Suarez and Messi). And they are, or were, 83% of the front 6 on the field vs Liverpool. Maybe you can do with 1 being old and slow (Messi), if the others don't share his flaws. But we had too many players who were old players with very poor movement, stamina and intensity. Only Vidal somehow makes the cut, but his age is visible as well late in the season, especially when you ask him to run even more than necessary to compensate for Messi and Suarez.
The problem in defense for me started with Messi and Suarez. Everybody concentrates on the back line because they don't think about football in modern, 2020 terms. Effort and coordination up front to block the construction of the opponent in deep phase is essential today. Our forward line didn't do anything to keep the Bayern CBs deeper, Alaba moved up on the ball at ease and created superiority in midfield time and time again. By this, your midfield becomes outnumbered very easily. Not only this, but a CB breaching our midfield allows a midfielder to do a forward run and still keep good numbers in midfield.
You need forwards who act as the first layer of defense, meant to stop defenders from the other team to be daring and create superiority, thus overpowering our midfield. Not necessarily press, but keep position well enough, and with enough work-rate and movement, to discourage the opponent's CBs from pushing up on the ball and breaking our lines. That's why Bayern used Perisic, not Coutinho in the forward line. Messi and Suarez not only didn't press, but Bayern CBs moved past them easily because they knew they don't bother to stop them.