To be honest I think with Nani we are in a fairly win-win situation.
If he is sold we win because we get say £20m+. We wouldn't sell him for much less than £15m, thus no complaints. If he is sold we win because we stop worrying about his potential to reach top level consistently, no longer our problem. We also win because a new player has the chance to shine, like Zaha, but this kind of balances out with the stop worrying about Nani, as we just have someone else to worry about!
If he stays we have the chance of him getting to his best under a new manager, thus we win.
It's so finally balanced that we can't really lose, he is not consistent enough for it to be a disaster if he goes, he has so much potential and at times having proved this that we don't lose by keeping him.
With Rooney on the other hand it is a much larger debate. For me if he goes we lose, but that is not to say it is not the right time to sell. We lose whenever one of our best players go, but no player lasts forever, thus picking the right time to sell is crucial and although we might lose it might be the best time to lose. Highest transfer fee we'll get for him, balanced off with questions of whether he is as good as he was and whether he is heading for decline.
Personally I would not sell either Rooney or Nani unless we have some seriously good signings lined up. We could afford to lose Nani and replace him with Zaha only if we seriously strengthen other areas of the team.
We could afford to lose Rooney and effectively replace him with Hernandez/ Welbeck/ Kagawa in the pecking order, only if we strengthen other areas of the team.
Basically losing Rooney or Nani is potentially less damaging than the positive impact of buying a world class midfielder. We have good strength and depth on the wings ands up-front. Yes wings is more depth, but all our wingers are at least of a decent standard, occasionally having world-class outings.
To summarise, if we can sign a world-class midfielder, which we should, then it is a less of a concern and we should just do what works best with the player. If we don't we need to get them to work best with us, as otherwise we will be lacking the collective qualities!
I should also add that if we keep both Rooney and Nani then we could potentially get by without a world-class midfielder and instead give our youth some opportunities, but that is a separate debate and is still questionable without Rooney and Nani getting back to their best.