You know, not any team could ever be perfect for more than 4-5 years 'in a row', we can't find such a team in football history, so i think it's a little bit fantasy to expect being top for several years.
Despite of that, not every coach has that much potential to stay many years at one club, you know how many 'managers' there are in football history that been staying long time at one club ? Ferguson and Wenger recently ( I admit that Ferguson is a damn good one lol ) but you see Wenger's team have been declining for years now, i think even players would be exhausted to see one 'face' for years and all of managers haven't that ability to keep players motivated all time, let's not expect Pep to be perfect at every aspect, he did get us use to see 'perfection' for 3-4 years, maybe for that our expectation of him became high over period of time.
Im sure if Pep had stayed more seasons ( one or two ) soon he'd have been fell down and fans start to saying 'Pep please leave us sooner to not keep declining'
And im expecting lucho himself will be like Pep as other managers throughout barca history been, it's hard to keep working at one of biggest club in football for years.
Let's face it, in modern football staying for a long time at one club would be 'honorable' and 'crazy' ,therefore a few managers could ever achieve it ( even in the past ).