Goalkeepers are invaluable, think of how much of a difference De Gea makes to United. Look at how they churned through any number of average keepers between Schmeichel and eventually getting Van der Sar. We've been blessed by an unbroken line of properly good/world class goalkeepers for nearly two decades now: Cudicini, Cech, Courtois. How different would our honours list look if you alter history and replace one of those three with, say, a Ross Turnbull.
You say that any old keeper could have succeeded for Man City, but Pep went for the wrong one in Claudio Bravo and it cost them in numerous matches during his first season. Yet getting rid of Joe Hart was still the right decision, as evidenced by his subsequent form for Torino and West Ham.
The homegrown player thing is overblown, we've already got 11 of them in the first team squad at the moment. Bakayoko's departure will free up one of the non-HG spots for Kovacic. If we end up winning the Nabil Fekir sweepstakes we can loan out Michy and keep Tammy on as 3rd-choice striker, freeing up another spot. Bringing in Rob Green gives us all the flexibility we need in this regard.
Pickford was only signed last season and judging by reports Everton were very intent on keeping him. Look at how they rebuffed our efforts to sign John Stones. The only way of getting him would be overpaying, so why not overpay for someone who could end up being a whole lot better. Butland was the best English keeper readily available, and I'm not wholly convinced he's good enough for a top Premier League club that wants to be competitive.