They're not even really 'splitting up the playerbase' in any meaningful way. Everyone's going ape-ship over the idea that the seas will be emptier, but all that's going to happen is that there will be fewer noobs on their first skull voyage to roll up on and sink while they're in the middle of Plunder Valley looking for a single Foul Bounty Skull. Or a random fisherman trying to catch their last Muddy Wildsplash. Or a Tall Taler on their fourth run through Wild Rose.
If your primary enjoyment in this game is to roll up on one of these guys and frak them sideways for an easy sink, no return combat, no loot and a handful of starter supps... I mean, no gains at all really except to ruin someone else's experience under the guise of 'SeA oF tHiEvEs, NoT fRiEnDs', you're actually just a Satchel of Richards.
They can't sail a Captained ship & progress Milestones, sail under an emissary & progress their monthly ledgers, sail for Athena's Fortune or Reaper's Bones at all, activate the Hourlgass, sail for a Guild, do live events like Gold & Glory or do the FotD in this mode. All to cap the main companies at lvl 40 at an excruciatingly slow 30% progression rate.
Either they stay in Safer Seas or they quit after getting bullied; they're not contributing to the ship count in Adventure Mode either way and you're not missing out on any 'organic PvP' on the seas no matter which way they swing.
But the liklihood that once they're gone through Safer Seas and gotten proficient at the game, they might move on to the normal mode is significantly higher than someone getting dumpstered a half-dozen times in their first session and staying in to keep having it happen over and over and over again until they 'git gud'.
What's your problem with other people doing that, really? how does that affect your play experience in any meaningful way at all?
Just because "WE had to do it that way, so they should too" or "It's always BEEN that way so it should always BE that way" is not an excuse or a reason.
Don't hate people on bicycles just because YOU had to walk to school.
(hate them for thinking they deserve a lane of the road when you're driving behind them)