Both surprising and not surprising:
Not surprising because all the point-to-point business passengers use GMP-HND. In fact, the ICN route has been all-Y for a long time.
Surprising because it basically means they are going to give up selling connections to US/Europe not served from HND, unless they completely open up the codeshares even to discounted tickets. HND-NRT transfer would only be worth it on a very heavy discount taking into account non-transfer of bags and long journey time between the airports.
Either way, the route has served me well during the peak of my NH travel to/from ICN-LHR and it will be strange to see it go after all this time.