The problem is this game also has the history, insofar as its advertising during its crowdfunding stage, of being a game with both online multiplayer, and offline single player modes, meaning that different people who funded the game had a different perspective on what they were supporting. There was enough grumbling when the offline mode was dropped altogether, with solo mode being a sort of compromise I suppose, a 'Well, no offline mode, but at least you still get to be single player!' The messaging has been quite muddled, but a good chunk of people in Solo WERE paying for a single-player game. Others, like me, bounce between depending on mood and general willingness to deal with the immersion-breaking presence of other players. (If they DID segregate the two modes, to be honest, I'd probably stick to Solo, and hope they'd allow players to make an alternate, separate character for Open.)
As for your last part, like I mentioned, unless it's a roleplay server, nothing can shatter the illusion of a grand space adventure more than a random commander messaging "hi how r u? =D " If I'm willing to talk to people on the Internet, I use Open Mode. If I want to just feel like a space commander, I go solo, because the moment someone who isn't EXTREMELY in-character intrudes on my game, I do not feel remotely like a space commander. xP
Again, if they do away with solo mode, two things will happen;
1) Many people will stop playing, which means less funding for the game.
2) Many of the rest will find a quiet and deserted corner of the galaxy, and basically it will be like they were playing Solo mode, only now they're irritated and less likely to fund the game further.
Removing solo mode will not suddenly force people to be more social with you. =P Even if they're sealed in the same star system, those who don't wish to talk to people just won't talk to people.