I can't stand the "you didn't name anyone therefore you're talking poo" brigade, be it players or managers. We aren't in the position to know everyone someone at the top of a footballing organisation would know about. For all we know Mourinho would happily have joined UTd, just needed to be asked, maybe Guardiola would have if they got to him 6 months ago, maybe Henyckes, Ancelloti, Hiddink, Rodgers, batman.
Players who you don't know are available, but a club makes an offer and the club agree to let them go. Uwe Rosler wasn't looking to leave Brentford but Wigan made the approach and he decided to go. Managers are no different, it's happened many times that a manager has been tapped up for another job and they've left. Who knows who Utd approached on the quiet, who knows who would have said yes if they did. Utd are one of the biggest teams in the world and titles aren't the only measure of football managers.
Southampton have yet to win a title but the manager and the chairman in particularly are being regarded as incredibly successful so far.
Nothing stopping a new manager coming in, saying the squad isn't up to crap and the league won't be a walk over like the previous season and he came in to entirely change the team. If after 3 years they have entirely changed the team and are challenging for titles, that would still be successful. As a manager it would be a bigger and more fun task(and likely more rewarding... not talking financially) to take a team, rebuild it and take it back to the top.
People will fawn all over Guardiola for winning the league this year... but my dad could have taken over that team, told them to change nothing and seen them win it. Taking Southampton and winning the league or even coming 5th would be hugely more impressive than taking Bayern and winning the league again.
There were probably 100 managers who would take the UTD job, some for financial reasons, some for the challenge, some for the experience. Moyes was the only one who really proved over a decade that he didn't have the mentality for the job.
Guardiola had a little experience with the Barca B team, maybe Utd should have been teaching a replacement how to be the next manager, may have made a mistake not doing that.