AMD have likely picked all the low hanging fruit that Jim Keller left them for Ryzen. I highly doubt they'll manage more than a 5-10% IPC increase again, before going to a new architecture. Hopefully I'm wrong though, their pace of innovation to this point (using Jim Keller's designs) have been refreshing.
That's what people said about Zen 1 , then Zen 2...
PS .. Keller Didn't design the core. No one person did.