They're not killing their kid.
Of course not. People will still be able to play Ever Crisis, watch Advent Children Complete on whatever service it's on etc.
But for the new continuity, with the new canon, people will have to play the Remake series. I don't think Square considers quasi-deleting AC, OtWtaS, DoC etc to be "killing their kid". If they had such concerns they wouldn't have made such a dramatic re-framing to the OG story the way they did with Remake Part 1's ending.
...It's also possible that they'll backpedal and have all the time shenanigans be erased from the Remake timeline, everybody has their memories partially erased and everybody only remember the OG version of events and then old post-OG happens as before. I can see them going either way.