Atalanta scored more goals than Barca last season without having a player who would start in Barca's attack or midfield. Messi isn't a guarantee for goals when you need them. But he is a guarantee that our pressing game will be worse than Liverpool's or Bayern's.
You and many others on here are guilty of completely separating Barca's failings as a team from Messi's non-performances in defence. It's like he is making a team full with mediocre players punch above their height and it is always entire their fault when they get smashed 3:0, 4:0, 8:2. It's always about Messi's brilliance when Barca win and always about his team-mates' mediocrity when they lose. Naive.
I predict we won't become a better team if we sign Messi, neither in the PL nor in the CL. There are upsides in signing him, but also downsides. Everytime we lose with Messi, fans and media will laugh at us. Liverpool will get a psychological boost. How come? Well, if we win the title, it's because we signed Messi. If they win it, they are absolute heroes. Media will have field days.
Barca need to rebuild their team but have no money. They will try to use Messi to get some of our talents. No way are we going to pay 200m in this crisis. So, Barca may want, say, Bernardo, Jesus and Garcia to give us Messi. Judging by the opinions on here, many fans would agree to give them these players for Messi. Or just Bernardo and Garcia plus 100m. For me, such a deal wouldn't be good.
That said, I'd like to see how we would perform if we lined up in attack Sterling, Jesus/Aguero and Messi with KDB and Foden/Bernardo behind them, especially against Manure.