Interesting indeed. His legacy was his unparalleled success, and it extended beyond football in the form of him being invited to lecture at Harvard Business School. Whilst United are currently nowhere near as successful as they have been in yesteryear, his legacy has not waned, but quite the opposite.
Sorry but that doesn't make sense, so we're not talking about legacy in terms of money inherited after someone dies but in his case the legacy of his "Aftermath" which is basically what happens after he has left not while he was there.
If Moyes etc had continued to have success and keep them at the top then that would be considered to be a good legacy that he'd passed on however as you say they are not as successful as they were.
On that basis his legacy isn't good but you claim it hasn't waned in fact the opposite, that's at odds with the meaning of legacy and the club results.
The fact he has lectured at Harvard and I believe other places about Management isn't a legacy and I always found a strange choice as his style of Management is at odds with how most modern business's would want to operate as it would break their ethics policy
His real legacy is "Fergie Time"
External Link/Members OnlyADDITIONAL SYNONYMS
in the sense of aftermath
Definition
effects or results of an event considered collectively
In the aftermath of the coup, troops opened fire on demonstrators.
Synonymseffects,end,results,wake,consequences,outcome,sequel,end result,upshot,aftereffects