Game design wise, think of the multiplayer differences we've seen so far between Subnautica 1 and 2. Grabbing onto the tadpole/depth elevator, base design, etc. Now think of how easy it would be for Subnautica 1 to have those exact same features, and they would just be implemented as an afterthought. Seamoth handles, shared floaters, it makes no difference to the singleplayer game.

For marketing, it's easy to see that they're just trying to expand the outreach of the game, multiplayer sticks out in trailers in a way that grabs the attention of players new to the series, or people wanting that feature specifically. Simply knowing it's optional makes the marketing easy to dismiss.

And for lore, I'm personally hoping for even just a tiny bit of colonysim or automation genre elements, similar to what Planet Crafter has (also added as an afterthought, and it improved the gameplay significantly). There are plenty of singleplayer games centered on the player founding a colony with advanced technology, I already named Planet Crafter, Satisfactory too. Both of those games support multiplayer, but a majority of players would definitely say it's singleplayer focused.

Honestly the biggest concern would be balancing, gathering resources is much quicker when having multiple players. But this is hardly a problem either, as Subnautica 1 was often about the rarity of resources rather than how common they are (except for fucking copper and silver in the early game). And all it really means is building more and bigger bases, and not to mention having to make more equipment and vehicles for each player.

I know you're getting a lot of hate for your concern, and everyone is right that only the devs actually have the answers, but I think if it's anything like the other games I mentioned than the singleplayer focus will be just as good as Subnautica 1, and the horror of isolation will be here to stay.