This is actually the biggest concern I have. In order for this game to thrive, it must deliver on the MMO aspect.
If you want people to play online, you can't do it by forcing them, or asking them to walk into a trap. Make the online All group a good place to meet friends and play, and people will want to join-in. (#1). You can encourage people to play online-ALL group by making it a good community to play with where people will chose to play in ALL group. Have player communities, have organized battles, have escort missions, etc.
But, If you allow it to be a cesspool of "shoot everything that moves" and "smash ships for LoLz!", with spawn-camping and dirty-language, then yeah people will leave.
This is what emerges when players have the option to chose All or Private or Solo: You have to interact with other players by mutual agreement; you can't force them to "eat dirt and like it" they'll leave.
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#1: It happened in Evochron Mercenary. because people in EM have the choice to play online or not, the online community is very welcoming. Punk players, bad attitudes, and spawn-campers are quickly eliminated. And yes there is still PvP, plenty of people want to duel.
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