I think Mrs Rob got it right (as usual). For a couple of years they spent nothing on the team, but Ferguson being the motivator he is enabled the tanks to run on empty for a while. Once he had left, the pressure was off as the players felt able to take their foot off the gas and have a breather. They had already won the trophies, so chasing titles was not as big an inspiration as it would be with a hungry team that was still trying for success. Without a manager able to motivate to the level of Ferguson, the only way was down.