Yeah the pursuit of Lewandowski made sense back then even when people were wondering why we were after a striker with Van Persie and Rooney in the squad. When you look back, Rooney getting dropped for the Real Madrid games were a big tell sign.
The thing was, Rooney getting phased out by Fergie was business as usual. Ince, Kanchelskis, Hughes, Beckham, Keane, Van Nistelrooy, Stam (although Fergie's gone on to say that was a mistake) all left the club to be replaced by other players who'd stepped up. Scholes and Giggs were the exception, rather than the rule. Gary Neville would've been given chances to do similar to those two, but unlike Rooney, he knew himself when his time was up.
In 2006 when Van Nistelrooy left, Rooney and Ronaldo stepped up. I see parallels when I look at Rooney now, and players like Martial, Memphis or maybe even Pereira. It's on Van Gaal to grow a set of balls and take a risk.