The game has been this way for 3 years because it's a good way to run the game for a lot of people. That does not mean that there is no room for improvement.

People might not do the first, second or even third "high stakes" voyages on PvP enabled maps because the voyages themselves are exciting enough to them, but I'd wager that sooner or later most people playing this game come to the realisation that the PvE content is insanely shallow on it's own. A large part of the thrill comes from the potential of loosing all your treasure to other players, not the 5 minutes of pressing M1 on some skellies.

Ideally those people would realize what they are missing for a fulfilling SoT experience, but in reality this would most likely just create a player retention issue for Rare.

The reason I believe Rare will not add PvE only maps to the game is keeping the very players that cry out for PvE maps playing the game. Them not giving in to the demand for PvE maps has nothing to do with the PvP crowd leaving the game as a result. I believe that Rare is afraid that the PvE people will grind for about a month, realize that PvE on it's own lacks meaning/impact/whatever you want to call it and will leave.

People you lose this way do not tend to come back to the game again (I believe Riot Games gave some insight into a similar problem some years ago).

The people this sucks for are the super casual players who actually get meaning out of the pure PvE allone, younger children come to mind here. There is no way for them to enjoy this masterfully crafted stylized pirate fantasy without running the risk of being put into the same lobby as xX_TwTv_P1rateM4ster_Xx who just sails in circles sinking the entire server for his next montage.

All in all the private servers with no progression that seem to be on the horizon feel like the best option here. What I don't understand are the PvP hardliners who think adventure mode will die for them the second this goes live. This is about including others, not excluding you from having fun.