The difference between Rooney and pretty much all the rest of our greatest ever players was that his decline came at an age when most players are hitting their peak, and shortly after signing the biggest contract ever given by the club - and although he can't be blamed fully for either of those things (especially the contract) it does add a different context to how he is judged by some.
Cantona promptly gave up football completely when he felt that he was past his best, similarly Gary Neville. Giggs and Scholes played for years more than Rooney and adapted their games so they could still contribute at the highest level, even if they couldn't play every week. Charlton, Robson and Keane all played for years longer and saw more of a natural decline (hastened by injury in the case of the latter two) rather than the 'cliff-drop' that we saw with Rooney. Denis Law moved on after injury, and George Best was (sadly) an alcoholic. I just don't get any correlation between Rooney's latter years (which some people view as 'terrible', and at the very least were disappointing in relation to his age, status in the team, and wage) and the latter years of our other 'legends' - they each have a specific and different set of circumstances.
EDIT: for the sake of clarity I should reiterate that I do consider him a legend of the club - I don't think it's reasonable to argue otherwise, but I also think it is pretty clear that the latter third of his career was a great disappointment, and can see why that - along with the contract disputes and public statements - have tarnished him in the eyes of many, to the extent where they won't personally consider him a legend even if his goals and longevity have made him one in wider terms.
Rooney had made 168 professional club appearances at 21, all for Premier League clubs. He also had nearly 40 England caps at that point.
Cantona had 70 professional appearances, all in France, and 15 of which were in the second tier. He also had at most 3 international caps. (He did have to do a bit of national service).
Neville had 69 professional appearances, and had at most 16 England caps.
Scholes had 56 professional appearances and no international caps.
Charlton had 86 professional appearances and at most 6 England caps.
Robson had 77 professional appearances, 19 of which were in the second tier, and no international caps.
Keane had 134 professional appearances, but 29 of them were in Ireland, and 10 international caps.
The only comparable one is Giggs, with 197 professional and 12 international caps.
Rooney has thus far played 755 games of professional football, at 31.
Neville had 687 when he retired at 36, Scholes had 784 when he retired at 38, Keane 743 when he retired at 34, and Cantona 485 when he retired at 31.
Charlton had 864 when he left United aged 35, and Robson 820 when he retired at 40. And obviously Giggs had over 1000.
However, by 31 Robson had fewer than 600, Charlton 600ish, and Giggs 640.
Rooney was doing at 16 what most weren't doing until they were around 21, and was doing at 18 what most were doing in their mid-20s. He's already played more games than a lot of players who played until they were older, and in terms of games played has already had a pretty long career. People forget he started at 16 so has already been playing 15 years.
Of the sticks to try and beat Rooney with, beginning his decline at 28/29 and dropping off at 30/31 is to be expected when you consider he broke through around 5 years before most do.