If SoT had come out as purely PvE game and had been exactly the same it is now, I most likely would never have picked this up. I was sold to this game after watching Zylbrad's goofy adventures on the four "seas" because the player interactions made it look so fun.
I don't think SoT would be special nor as enjoyable as it is right now if it was purely PvE game because the PvE game play loop in SoT isn't anything special, go on a voyage, do task(s), get loot, deliver it to op, get gold that you can spend on cosmetics. Quite dull if the only thinks you could do were mindless grinding which becomes boring after hundred vaults, trust me, I've done it, or "getting better" by becoming more efficient chest tracker.
SoT was build to embrace player interactions and that's pretty much what all the treasure is here for, and why only real way to get better is by learning instead of upgrading ships, only thing making encounters with others, was it friendly or hostile one is ones goals and "skill level", and deceptive plots they twine for you.
TL:DR, would be boring and no where as successful as SoT was made with player interactions in mind, were they hostile or friendly, and for SoT to work as purely PvE or PvP game there would have to be plenty of different game play choices to the point SoT wouldn't resemble the game it is and has been.