I think EPIC might be being a little ambitious in thinking that in only a couple of years there will just be ray-tracing in game engines. There will probably be some form of hybrid ray-tracing effects in most game engines by then, but the big switch from rasterization to ray-tracing isn't going to happen that quickly.
It was something we spoke with David Wang about too:
“I do think ray-tracing has its place, but is this going to be a sea-change overnight? Will people compromise on 4K display performance and start to become jaggy? I don’t think so.
"So, I think it will take time for this sea-change to happen, but even in the end I still believe it will co-exist with the conventional rasterizaton technology. Just because frame rate and resolution are so important to people.”