Pep joining City has more to do with the lack of planning/ insight by other clubs, including United (this is the excruciating part) because United can offer him as much money as City can given the club's enormous revenues. We were fully aware of the fact that Van Gaal wasn't going to be part of the club's medium to long term plans - and we should have stepped up our efforts in pursuit of Pep/ Carlo/ José (at different times through the season) - who are all upgrades on the Dutchman - who is no longer the visionary coach he was at Ajax, or even Barcelona. Instead, we're doing exactly what we did when Fergie retired - scoffing at the likes of José for a multitude of reasons that have been addressed elsewhere, or waiting until the others are snapped up by other clubs, before appointing the 5th or 6th choice - which Giggs would represent, if indeed we do appoint him. It's like we're not even trying for Guardiola (both according to the brief, and the likelihood of him joining City because of their background interactions since the summer) - which is rather sad, and grates on the more pragmatic section of fans who want to correct our course before we fall further behind the leading pack (not just domestically, but European football as a whole). Once you lose the winning touch, it's hard to replace the aura - and we've already lost a lot of our sheen since Fergie hung up his managerial boots.
If United were serious about appointing Pep, we would have contacted him in the summer when it was public knowledge that he was entering the last year of his contract at Bayern (maybe we did, but it's highly unlikely given the narrative surrounding Van Gaal). We would have sold him the vision of United's 'project' and how we was going to fit into that, guaranteed him the tools to succeed (in terms of transfers and more guaranteed money throughout his tenure), and laid out the red carpets by offering him a massive contract. That's what City have done (presumably) - they've been preparing for years, building up their club from scratch (while we stood pat in terms of infrastructure and background staff); and now they're at a point where they need to take things up a notch, and become a serious European contender, while we're regressed from not just our peak, but even median levels over the last couple decades. Instead of planning an upheaval to move on from the Fergie era, we're busy reminiscing and content with looking at others from our ivory towers and harping about how 'We're United' - as if we have a divine right to win and they'll come because of our name and history, when the landscape of football is changing rather rapidly in front of our eyes; and a lot of players/ managers these days don't give a shite about stature (unless you're Real Madrid and Barcelona) - they care more about present circumstances, money, and chances of success.
Ultimately, we've got no one to blame but ourselves - this is a result of our antics in the transfer market for a long time, the fact that the owners didn't invest back in the club (stadium/ academy/ background footballing staff) and our deluded/ erroneous decision making post Fergie. We botched the transition from him, we don't have a discernible template for the future, we're not willing to sack a massively underperforming manager because apparently he's laying the foundations - which will necessitate a 3 year drought where we win squat (even though a lot of top managers have achieved far more with far less, and in a shorter timeframe) - and that'll be followed by a cycle of Pep at City, where we might down tools and win squat again - because we lack the wherewithal in terms of top players guided a a top manager at the helm - all because we plagued with indecision, and consumed by hubris, and our misguided sense of invulnerability - and the fact that we're likely going to overlook Pep's kryptonite for intangible reasons (when he's been angling for the United job for years).