Games Workshop and Forge World are obviously setting up a replacement for Horus Heresy but I for one doubt it’s ability to take the roll. I love the Old World but I really don’t want to spend endless amounts of money on beautiful campaign books for a super crunchy game system.
If it were Warmaster scale I might be tempted, but that would limit their ability to attract non Old World gamers to buy miniatures. Even if an AoS player doesn’t play they will still buy minis to use in their army. Same goes for Horus Heresy.
I know everyone thinks Primaris will fully take over regular space marines but that would leave a lot of Heresy players with useable armies and no 30k support. We already see them sprinkling in 30k tech into 40K (Neo-vulkite, dreadnoughts). I think the same philosophy will apply the Old World.