At least the appearance against Juve gave a little hope. They performed quite well although this cannot describe them in the League or UCL matches. Barca without messi will still be able to compete. Although I believe what you say is true. They are in the process of regeneration, starting to renew the golden age with messi who has left.
At least Koeman has this task, he has to think smart with the current players. Financial problems seem to be preventing Koeman from shopping new more great players.
Barcelona in the last couple of years, gave hope to their fans this way. They are more similar as Arsenal but at higher levels. They are strong most of times in a season but in important matches, they lost and all other good results in are non sense generally.
Yes, even though it was only a friendly match but it looks like both teams played quite seriously too and yes, the big win can also be a motivation for the team in the first match later. Basically a team must be able to play with high cohesiveness although indeed, sometimes in a team there is at least one influential player in the match but this cannot be relied on forever.
Clubs are in ending of their pre-season trainings so such friendly match are last chances for them to correct their players and tactics for the coming season. It is understandable to see they played the friendly match very seriously. Barcelona did it better than Juventus and brought a little hope to fans.
With the departure of Messi, I still hope that Barcelona can perform well, because if I look at some of the history when Messi is unable to play due to injury, at least it is enough to make Barcelona unable to perform consistently. Now it's a different situation because Messi has really left, and the team has to move on and fight together while keeping the team together is something that is important.
You are right. In the past season, when Barcelona have to play without Messi, their results are not too bad. With Messi, they can not win La Liga and are struggling to end in top 4. Without Messi, I believe they will be able to get a position in top 4. It means the difference is not dramatic.