Well Michael Keane and Nick Powell both went out on loan. Tom Lawrence was sold to Leicester. Of the senior players that had come through the ranks, Cleverley was loaned out (later sold) and of course Welbeck was sold to Arsenal. Januzaj and Lingard were loaned out the following transfer window.
The only reason why Di Maria and Falcao left so quickly is quite simply because those signings didn't come off. However, at the time they signed United fans were excited and I don't remember too many, if any, complaints about the fee paid for ADM, the rumoured wages you were paying Falcao, and the moving on of several academy products (Granted, Welbeck's sale did generate a few muted grumblings). There were even banners at Old Trafford referencing LVG's Galacticos.
As for City, I think you'll be surprised over the next few years because while there will certainly be big money signings, I can see Guardiola promoting a good number of academy products to the first team as well.