That goes without saying. But by working hard he might be able to extend his playing career a little longer, which will mean more cash.
Its even possible, if you think about it, that a pay cut now - as long as it wasnt too extreme - could help facilitate a move to a club where he could play regularly, thereby improving his performances (we all know most players do better when they play regularly, and Rooney more than most) and extending his career for a few extra years, which would mean more money in the long run. Conversely, maybe milking his current United contract until the end, but languishing in the reserves, means he will fade into obscurity and end up attractive less lucrative offers next year (or heaven forbid the year after).
Also, if he just hangs around doing nothing, collecting his salary and not playing, does that jeopardise his relationship with the club? Does the club definitely then take him on as an ambassador once his playing days are over? I assume so, and in fact the original reports made it sound like that was already all agreed and in place. But you have to assume Mourinho and Woodward will be hugely frustrated paying him all that money, it might create some friction there.