First of all I don't think Xavi and Thiagos situation is really the same. Xavi started to become a regular in the 99/00 so he was a few years younger than Thiago is right now. Also Xavi didn't have to replace such a great player. No offence to Guardiola but Xavi is an all time great CM, even among the best and still among the best CM in the world. Xavi can talk to him all he wants, but what can he assure? That in a few years it will be his side? That he had to wait too? Thiago is an amazing player and deserves to start week in week out. Except for the FCBs that would be the case for every team in the world.
Also did some think of the consequences that the Thiago situation could have on the team? Right now this is the team of Xavi/Iniesta/Messi. Let's say that Xavi is slowly showing his age and simply doesn't perform THAT good anymore and Thiago plays stunning for you. It would create huge pressure on your manager who to play and I don't think Xavi and the locker room would really appreciate it if Xavi would be benched in the big games. After all it is still Xavi, but what message would it give to other younger players? That your performances don't matter and because of your standing in the team and reputation you make the team? Not even to mention that you still have Fabregas! Maybe he can finally adapt to playing CM for Barcelona. We had a comparable situation with Pique at United. We knew his talent and how good he is but honestly Ferdinand/Vidic were simply too good to break up. If he'd be 2-3 years younger everything would be much easier for Barcelona.
That's why I think BOTH clubs could benefit from a deal here. We let him play week in week out and develop his amazing talent even further. He becomes a global star and returns in 3 years for you. I don't think even if we get him that we'd him any longer than max. 4 years if he really develops the way everyone thinks. After 2 years all the Barcelona player at the Spain NT would talk to him to come back. He loves the city and the club. He'd simply do a Fabregas and join you guys in 3-4 years again. We couldn't do anything against it and because he'd be probably in the last years of his contract he wouldn't cost the world. Of course we'd make a profit on him but I'd believe he'd go well below his market price like Fabregas.
Of course the best solution for Barca would be him staying and simply be happy playing 2nd fiddle to Xavi and just accept that in the next 2-3 years he won't play big games as long as Xavi is injured, but don't hold it against him if he leaves. We United fans were the same with Pogba but just look at his success. It was the right decision for him to leave United and I really believe that the same would happen for Thiago if he joins us.
One thing I think you UTD fans need to understand about Thiago is that the Thiago version that you saw in the Euro U-21s won't be the same player you get once he starts playing in the PL and CL. He looks like a god amongst mere mortals whenever he plays for the youth national team yet he doesn't do the same things as consistently as he does there on the higher level.
Don't think that I am bashing Thiago, he is an amazing talent and for his age, he is probably the best all-around MF. However, he has been pretty inconsistent when he plays for us and this is why he hasn't benched Xavi yet. The Thiago U-21 version is what you will get in about 3-4 years time where he just bosses games like its nothing. He doesn't quite do the same thing when he plays in La Liga or in CL based on his performance last season.
However, with what he has shown recently and Xavi's decline recently, next season should be his breakout year. This is why, if he is wise and really wants to play in the WC as bad as the media claims he does, it makes more sense for him to stay here than leave.
Leaving Barca won't change the fact that on the Spanish NT, he will still be competing against Xavi, Iniesta and all the other MFs who are in the team now. As I have said prior, I am skeptical of the assumption that he will magically light up the PL and I think Barca's style is much better suited to his strengths.