Some counter-points:
1. As you say, it's hard to think of a big-money player who's been bought for solely commercial reasons. But then there's never been a club as rich or as commercially driven as United before. Also I'm not saying Rodriguez would simply sit on the bench and appear in Spanish-language adverts. He'd get plenty of minutes, but probably wouldn't start in the very top games.
2. We don't need to recoup the money from him directly. If he locks down the Latin market for us we'll be coining it in for years.
3. Again, it's pointless to say 'this has never happened' as football transfer fees have spiralled so quickly. We're about to get the first £100m player and that could soon become the new normal for a top-tier talent. Our own Jesse Lingard has shown that in-and-out players can now earn £100k a week. Why can't they cost £40m?