I have nothing against Guiu, it's his career and he can do whatever he wants. I don't have the right to blame players if they look for financial security over development at a young age. I can understand their decisions because you never know when you can get seriously injured and your career on top level is close to over but doesn't mean I have to agree with them.
I've never wished injuries even to the players I disliked. Won't even talk about death wishes that I know some players are getting. That's just sad and disgusting. At the end of the day football is just a game. There are far more important things in life. Some people are taking things too far, especially in this stupid social media era.
So in short, I don't wish Guiu bad but I don't think he'll succeed at Chelsea either.
First because I don't think he has enough quality and Barca doesn't have history of producing good strikers. Lot of La Masia kids were hyped after a short breakthrough period in first team but Icardi (who never played for the first team) is the only one I can remember who made a good or maybe even great career in a top club and I would be surprised if Guiu would get close to that. He'll probably be closer to the Munir or Sandro level long term.
And second because Chelsea are a mess and just hoarding talents without a clear plan what to do with them. Maybe time will prove me wrong but I have every reason to be sceptical about their transfer philosophy until they will actually have some results to show that their approach works.
Guiu will probably be one of the 20 players training separated from the first team within a year or two (after they'll buy 15 new players in the meantime) and then send on several loans before being sold to a midtable PL or La Liga team.
That's probably his ceilling (and wouldn't be bad if he would actually make it to that level) but going to Chelsea at this stage of his career will also limit his development imo. Would give him better odds to succeed longterm if he'd go to a Leipzig/PSV/Lille/Brighton,... So clubs that would actually have a good plan and present him a path to the first team minutes in future if not already this season. I'm sure Chelsea gave him some promises as well but how that will work out we'll see. They probably promised first team football to Datro Fofana last year too. And where is he now? Just loaned to a newly promoted Turkish club.
If Guiu believes going to Chelsea was the right step for him then so be it. Barca will survive without him. He's far from the best player or biggest talent that has ever left this club. But only time will tell if Guiu will survive in a cruel outside world. Grass is not always greener on the other side. Most former La Masia players already have that experience and I'm sure there are more than just a few of them who also have some regrets.