No one knows. A lot of folks will claim to, and present various theories in various stages of baking (mostly half-baked), but it is all guesswork.
I'm not even particularly convinced GW folks know.
If someone up top says 'Those made us a lot of money, do some test kits and see how well they sell alongside they new stuff;' GW will do that. if they feel like they aren't leaving money on the table by continuing with the current oldmarine kits, they might let them go instead.
Even if they are officially 'retired,' nothing stops them from bringing them back if the money is right.
Its a business decision, and there's nothing at all dictating GW do one thing or another.
Though since GW believes Marines make them more money than anything else, if the decide to End Times oldmarines, a 'Warhammer: the Old Marines' project would happen a lot faster.
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