1) Because RVP has a pretty dire record of injures and since you are buying him in his prime for a significant amount of money... Your hoping he does not get injued in the net 4 years and performs to the best of his abilities as you have no way to recoup your outlay
2) Pretty sure there are younger, cheaper players out there who could have provided cover for Rooney if that was the aim of this transfer. There are none better than RVP surely but if depth was the problem...
3) Baines is a different matter though... He is not as injury-prone and would tie in nicely with Fabio's development. He can take over the starting spot from Evra next season and then when he starts to decline. Fabio should be ready to assume the starting role. Hence why he is a perfect option
Rules you out of getting quite a few top players who could have plugged those holes well (Fellaini, Moutinho)
Also regarding the Oscar transfer, 99% sure it was in Euro's and not Pounds ... Does not matter though. Its irrelevant to the debate