Please, here me out as I play the Devil's advocate and say that this won't split the player base. In fact, it will bring it back together. Let me bring up a few points as to why I believe that.
1.) I absolutely here what the current player base is saying with the worry of splitting the community. Yet, as of now, one can say the player base is already severely split, with the majority of PvP players gone and playing other games. This mode will inevitably bring back those players. Those players will now have a reason to load up SoT and play it. They will grind up the new 'Sea Dogs faction' in whats being quoted as 'shorter sessions'.
2.) 'Shorter sessions' was something Joe Neat said today on the Xbox panel. This most likely means queuing into a lobby for a timed PvP session, and that Pirate Legend is reserved for Adventure Mode. This will see more PvPers trickling into the adventure mode to get Legend and other time limited cosmetics during event DLC's. Furthermore, adventure mode cosmetics were seen in The Arena trailer. Meaning PvP cosmetics will be used in Adventure. If higher level PvP cosmetics require Legend, then that will also bring more people to Adventure and vice versa.
3.) The faction leader is seen playing with a silver coin and saying, "Competition runs through your veins... and who am I to disappoint?" - This leads me to believe that there will be a new currency: Silver Coins(?). My guess is they will work similar to Doubloons, allowing you to buy PvP only cosmetics. We can only speculate how it will work. Yet, Legendary PvP cosmetics will bring more people to Adventure, as they will have to grind.
4.) PvE and PvP players alike will now be able to que in and test their skills without any real consequence or need to search the sea for hours. It will be a place to practice, and get better at PvP with the click of a button. It will scratch your PvP itch on a moments notice if you ever had or have one. You will see the birth of extraordinary PvP captains that will inevitable be lured by the luxurious cosmetics only Legends can get in Adventure mode. Maybe you will find a captain that you can sail under, or become one yourself.
5.) I also understand this may bring back more toxic players, but even Micheal Jordan was known to be the biggest and most toxic s**t talker on the court. It's the psychological part of competition, as they are trying to get in your head and doubt yourself. Still, this mode may quarantine the more 'toxic' PvP players out there. Allowing more friendly PvP interactions in adventure. Again, not a bad thing for the current player base's worries.
6.) You will be attacked and pirated in Adventure mode. That will not change. In fact, the appeal of legendary cosmetics may only serve to populate the adventure mode with the incoming PvPers.
7.) More players is not a bad thing.
I feel this mode addresses a lot of the concern from a huge portion of the player base that has felt neglected: The PvPers. To say this mode will split the player base can't be true, when most PvPers have been gone for a long time already. This will in fact grow the PvP base back to what it was, and those PvPer's will face the inevitable question of if they want to hit adventure to 'get legend'. And when some of them do... well, 'be careful what you wish for'.
Much love!