The diff with Utd under Ferguson compared to now is at the time they were THE team all players wanted to go to.. they could out spend whoever as well so would bully teams into selling them their best players. Before Chelsea and City (and the increase in the money coming into the prem in general) it was very rare a player who Utd wanted didnt go there and stayed/opted to go another team.
Nowadays they are having to spend vast sums of money on average shite cos the great players have a choice of 3/4 teams in the prem alone.
That, added to Ferguson purchasing players for his last swansong rather than the future of the team it's what's left them in the mess they are in.