SAF was an exceptional manager who got the most and more out of an unexceptional squad last year with the underperformance of the teams around flattering their position. Like all people on top of their game, he also knew when to retire, so he left at the top.

Keane and Scholes were never replaced, Giggs and Ferdinand are the end of their careers, Vidic is past his best. If you were building a top fantasy Premiership team only Rooney, De Gea and Mata would stand any chance of being in the squad, now compare that to 10 to 15 years a go and how many MU players would have been in it.
Give the Moyes a chance to rebuild the squad and then if it doesn't work get rid of him. That means at least giving him another season and if it is a better one, then a further one or more beyond that providing money continues to be invested and they improve. Sacking managers at the first whiff of failure does not build stability and long term success. It has taken Liverpool several seasons under Rodgers to rebuild and to get where they are.
The Glaziers taking over MU has cost them £600m to £1bn that over the last 10 years which could and should have been invested in the squad. SAF is the only reason that this 'crisis' has taken 10 years to develop. Liverpool fall came much quicker under the last US owners. This summers transfer window will be critical in terms of the funds available where they need to spend at least £200m and with no Champion League football being able to attract players of sufficient quality will cost a premium in terms of transfer fees and wages.