I do agree that the game's story/atmosphere should be designed for singleplayer first, since that's what drew a majority of people to the first game.

However, I think MP could still retain some sense of atmosphere as long as they have a good in game voice chat and keep it proximity based, like in Lethal Company.

It isn't scary at all for your teammate to tell you exactly where they just died, how, call out the leviathan heading your way, then continue with a joke they were making. But having a friendly conversation over the radio cut off by their violent death to horrors unknown could be scary, even if facing the unknown alone is fundamentally scarier.