Just curious, but what is your definition of 'core cast'? Do you mean from the first episode? There are still quite a bit from 1st season. Willa Holland, Paul Blackthorne, David Ramsey, Emily Bett Rickards and Katie Cassidy. Sure, she's not the original Laurel any longer, but the actress is still going. Also, Even though Arrow hasn't been as strong as it used to be, I still enjoy it. Would hate to see it go, even after next season. But I do see what you are saying. However, if it does get cancelled in the near future, hopefully we would see guest appearances from the Arrow cast, especially during the crossover events.
On another note, not sure who has seen the recent episode of Black Lightning, but there was a reference of "Supergirl" made in the episode. It could just mean Supergirl from the comics/movies in their world, but I find it funny that they indicated that BL would not be part of the Arrowverse. Could that be changing and maybe some foreshadowing? :)