I liked Peter, but can see why the actor wanted to leave, as his storylines have ben repetitive and a bit boring lately. He's good looking and can act, so should get more work if that's what he wants. After so long in a popular soap he's probably well able to retire and be comfortable, and I wouldn't blame him if he did that.
I agree that Simon has been a non-character. He was a cute child (although miscast) but as he got older he's lost his cuteness and not replaced it with acting ability. Chesney was also impossibly cute, but is much less so now, although he can act. David is a very good actor (and the same applies re looks) and Amy is good too, although has kept her prettiness as she's got older. On the whole, the casting team are good at finding talented children who can stay the course, so getting it wrong with Simon isn't a bad hit rate, really.