How is it "diluting" the game modes especially Solo? Do you think that Private Groups are also a problem?
In the case of Solo mode, the "dilution" is that your efforts in this Solo universe are still rendered naught by the Open mode, since the universe in Solo and Open are one and the same. Even in Solo mode, you can not decide to try and push faction X to expand, unless the majority of people in Open currently want to push the same faction. If the people in Open push faction Y, you can try and job for faction X all you want and they'll still lose influence, and there's nothing you can do to change that. A "proper" Solo experience should be unaffected by what other people do in other modes of the game.
Ideally, each Private Group should have their own universe as well, only affected by this group, neither from random people in Solo, nor random people in Open.