Rooney has always been a patchy player who has had one good season followed by one average one. His position was clearly usurped by RvP and he has had to play behind him for the majority of season where he isn't a natural fit.
He is only 27 years old and, easily, has another three to four years of top level football in him. A decline is what has happened to Torres and I don't see anything similar happening to Rooney in near future.
One reason why Man United are looking to dump him might be his huge wage bill. He is earning some 250k per week and United had offered him this salary after his last tantrum when they couldn't afford to let him go (to City who were tapping him up). But, now that they got RvP in with similar wages, they might find it a bit difficult to have two players being paid that much.
In any case, his wage is one of multiple reasons why we are never going to get him. But, saying that he wouldn't be a good signing here when we seem to be praying for Jovetic, Higuain
or Benteke is far from truth.