I agree that it is only one of the options and that it is not the most important but surely it is the hardest? As it have every single threat that solo has but with the addition of other players, who can be the most aggressive and skilled opponents in the game by a long way. I play 90% in open because I enjoy it but sometimes I pop into solo, basically because it is so much easier. I normally do it when I am watching a film at the same time and just want to get some stuff done which doesn't require me paying that much attention.Open is but one of the play options. It is not the most important, nor the hardest.
I think the whole question of PP and game modes is a tricky one. PP at its core has a lot of functionality which is aimed at cooperative / faction vrs faction play. Solo mode allows you to bypass that. I am not sure what the fix is though.