I researched a bit about this GameCore stuff and turns out: Everything is pure speculation. MS hasn’t even confirmed the existence of GameCore at all. I bet if there’s a new API for Xbox, Epic is working on integrating it as long as MS is more open about everything. The primary reason for missing first party support from Epic is Microsofts policy. Our Lord and Saviour Tim Sweeney stated multiple times that UWP in its current shape sucks.
Long story short: Don’t panic 
MS will very likely have a plan for upgrading or something. There are so many XB Live Creators Program users, they’d shoot themselves in the foot by abandoning all of 'em.