Oh it won't turn us into a shit club. But it would strengthen Chelsea hugely. They finished pretty far behind this season, for many reasons: they had new players discovering the PL, they had a mini meltdown during the first semester, Benitez has been favouring the cup competitions, I feel, to a certain extent... But next year, if they do have Mourinho back (or any other sensible manager, who are able of managing a squad during a whole season), and with a couple of inspired signings (i.e. a defender and a striker who can finish), they'd be a force to be reckoned with.
When Falcao was mentioned on these boards as a possible recruit for them, people were saying "they'll be immense, that's all they need". Surely recruiting Rooney, a proven PL striker who will have a huge return of goals for them when playing in his preferred position, would give them a huge boost. I'm not saying they'd be absolute favourites to win the league, and that our team would be shit compared to theirs, but Rooney in the n°9 position is one of the best players in Europe, and for me the best they could hope to sign.
I'm not in doom mongering mode or anything, it just seems like common sense to recognize that selling him to them would definitely hamper our title chances, regardless of all the other elements of context (the retirements, the new manager...).