My take on this is I'd much rather have Cortois, DeGea, Lloris or Forster, but I wouldn't swap him for Guardiola's pet project.
He's still young for a keeper. He's mostly solid & pulls off some great saves. Yes, he looks like he's got a mistake a game in him (sometimes two), but look at DeGea when he arrived at ManU.
David Seaman went through bad patches. Joe Hart is better, but older & still prone to brain farts.
I trust Koeman & his team to know potential when they see it (or not). I don't see Schmeicel or Hart as a significant step up. Maybe there's a top keeper somewhere, over 35 & available, who could come in next season. Give Robles time to develop. I'd hate to bin him off & see him turn out like DeGea somewhere else.