Evidence towards:
There were some SW assets that aren't implemented into the game, notably the Quackening Ram, a community idea.
These leaked on Stream and were quickly swept under the carpet.
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Community support for DST:SW - Though I personally believe it can't happen for lore reasons, there is a clear support for it within the general community, if not the forum community.
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Don't Starve Together: Reign of Giants - If one DLC was ported, why not the other? seems to be the general consensus, and it does have some merit to it, as it shows they have the ability to do it, perhaps even if not the time.
Any more evidence? Post it!
Evidence against:
Klei: We currently have no plans to port Shipwrecked to Don't Starve Together + JoeW's post - Paraphrased as I don't have the post, but it effectively mentions how they will not say "Yes" or "No" outright, after how MP was a "No" then got released. Though, it seemed to be clear he was leaning towards "No." without saying it, in that post. - For example, ONI:SW was mentioned, to a resounding "No plans for that."
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Bugs! Creepy-crawlies and code errors - Shipwrecked is more or less renowned for being full of bugs. Personally I haven't experienced many, but there have been a lot of potentially game-breaking bugs, however, usually related to things only in Singleplayer
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Shard issues - Shipwrecked logically would require two more shards, Shipwrecked & Volcano - This sucks especially seeing as the Volcano is so tiny, yet would require so much more. Already Klei combined the Caves & Ruins into one, making SW like this wouldn't work for most of us.
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Lore - If Shipwrecked was combined with the main world, and potentially the Volcano with the caves, this still leaves a lot of plot holes. Why does Charlie want it back? - This isn't even counting the holes it has within itself, such as the Shadows with no Ancient basis.
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Processing Power - Shipwrecked requires a much better PC than Vanilla Don't Starve/Reign of Giants due to it's Sea & more hazardous Weather effects. For example, Hail with a bunch of falling prefabs, or Dry bombing everything.
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Seasons - If SW was to be combined, what seasons would it use? There's no real good answer, they'd conflict with each other.
If you have any more evidence against, post it!
Community Ideas
Anything you'd like to see in DST:SW? Might as well post it below!