I say we'll get to know whether I am or not if Pep arrives.
If someone said in 2008 when Pep arrived to a team ending 3rd behind Villarreal on 67pts the year before that they'd win the treble immediately, and that they'd do it by selling Ronnie and Deco and bringing Pedro and Busquets in, you would probably call him delusional too...
No guarantees for trophies obviously, but I am sure that Pep would show I am not delusional, if he arrives. I think Xavi or Pimienta can do it too, but I am not as sure.
There are quite a few big differences between 2008 and 2021.
1.) In 2008 we've had a 28 years old Xavi in his prime years after just winning Euro. We don't have a midfielder like that now and it's very unlikely we'll find one. And I'm not talking only about Xavi's football abilities but also his experience and leadership on the pitch. I fail to recognize a realistically available player who could provide us those things immediately even if he'd be an inferior player to Xavi based on his technical abilities/passing skills which I know you give a lot of importance to. I don't though so I'd take someone like Wijnaldum short-term even if he's on the wrong side of the 30s but he has good chemistry with FDJ.
2.) Messi was 21 and on the upswing of his career. He's a different player nowadays. Still great but he's been on a physical decline for years which is showing especially against better opponents. Even if Pep would return Messi wouldn't score 80-90 goals per season again.
3.) Before Pep coming some older players in that team have already won a CL under Rijkaard so we've had experience but majority of the players were still motivated and hungry to win more (and those older ones like Henry came here with a goal to win the CL). Nowadays our veterans are mentally and/or phisically drained. They don't have anything left in the tank which was shown again and again against better teams in Europe. They can still look good sometimes against worse opponents but even there they're not consistent anymore. Or we could say they're consistently inconsistent.
Sometimes we could get away playing at 70% against teams like Granada and Levante because of our much superior quality but nowadays if we don't give close to our best we'll struggle to win. Those teams still consider Barca as a big team and are giving 150% to win but at the same time they're not affraid of us anymore. Long gone are the times when teams were coming to Camp Nou with a white flag.
4.) Should've probably put this above as I think it's the biggest reasons for our downfall. That 2008 team was stacked with leaders and players with character. Puyol was the best captain I've ever seen and having a prime Puyi around alone would make a world of difference in our recent seasons. But he was not the only one. Marquez, Abidal, Alves, Henry, Keita and obviously both Xavi and Iniesta were all captains or vocal leaders of their clubs/NTs at one point in their careers. Now our captains are captains only on paper based on their seniority. We have Messi who is GOAT as a player but was never a great leader. Busi stopped being a player 3-4 years ago (and should've been long gone) but was never a leader. And better not even talk about Pique who wants to be seen as a serious businessman besides being a part time footballer but deep inside of him he's still a child.
Sure, there are also some similarities that we could build on (at least our hope) but they are far from certain they'll turn out as good as we wish.
1.) FDJ is 24 as Iniesta was back in the days. He should enter his prime years though he's still inconsistent at times but a problem is he doesn't have a more experienced "partner in crime" like Xavi who he could work with and build a great chemistry with.
2.) I usually hate to compare youngsters to proven players, especially legends and even less so to GOAT like Messi as this is unfair to great players and also put more pressure on the kids. Everyone should build his own career and be known as the best (insert random name) player in the world, not being the second, third, fifth coming of Xavi, Iniesta or Messi.
That said, pre-injury Fati was in many eyes seen as someone who could potentially take over as a "face of the team" and our key player once Messi would eventually leave/retire. But now Fati's future is more uncertain. We don't know if he can overcome his injury problems as a youngster like Messi did (and I'm aware Messi's injuries were different than Fati's) and still make a great career or he'd go down the Umtiti/Dembele road. We can only hope for the best because our hopes for a better future would surely be much higher with Ansu fully recovering and start to fullfill his potential.
3.) We also have some other talents besides Fati who we could feel more or less good about(and fans have different opinions about them which is fine) but they're just talents at this point. There is no guarantee even one let alone more of them could make a similar longterm career here than likes of Pedro or Busi. We could probably be happy if we'll get someone on a similar career path to Roberto in the end.
So assuming Pep would return this year he'd have a lot of work to do starting by clearing out the veterans and replacing them with younger players who would at the same time also need to bring more personality to the team. He realized already back in 2012 that team was close to exhausted and he'd have problems to continue to motivate players who have already won everything had he stayed. And some of those players are still here almost a decade later. I doubt he'd look forward to work with Busi or Pique again just for emotional reasons (Messi would be different case because he's still playing on a high level).
If he'd have to come in and replace at least 6-8 players which is what I would expect he'd want to do (and we can't financially afford that which is another reason for him not returning; also, I'm talking about replacing players who have played important minutes this season, not likes of Puig or Matheus Fernandes) we then probably couldn't talk about the same squad which you consider as "world class, maybe the best in the world" anymore, right?
But Pep won't return so important question here is can Xavi do what's needed (with a full support of Laporta, Alemany&co.), come in and say goodbye to most of his amigos or at least give them lesser roles in the team to stay around for another season which is more likely? I seriously doubt it. But if he's aware that we can't move forward as long as Busi, Pique and Alba are unquestionable starters with no competition then I'd welcome him back. If he can't do that better for him to stay away from the club for a few more years and stick with Koeman or bring someone like Blanc for all I care. Won't really matter much because these are the players who are dragging us down regardless on who sits on the bench or who we'll manage to sign this summer (probably just a few players on a free anyway).