I actually can grok the idea of wanting to separate progress in 'open' from progress in 'solo.' No, I really can. I think it's a fairly silly concern in a game like Elite: Dangereaux*, but I can understand it. And it may be unfair of me to consider those concerns 'silly,' but at this time, there's no real utility to having the subfactions/minor factions increase or decrease in influence. There might be faction allegiances in the future, and concrete advantages to one minor faction being ascendant or dominant in a system. I'm still not convinced, however, between the P2P networking structure and the limits on instancing, that even then there would be any worth to splitting out the background sim. As I see it, FDEV is all but deliberately hobbling the ability of players to form the sorts of stars-spanning corps that you see in EVE -- and I'm okay with that. This isn't a game about territory control; they don't want it to be, and a 'lot of folks' don't want it to be. (Take that with a lick of salt -- it's some nontrivial number significantly greater than 'zero' and significantly less than 'all.')
But again, I can understand this desire, so I'm not against ways to make it somewhat more... I don't know, equitable? Less hackle-rising at least. I don't think that splitting the background sim is going to do it, nor do I think limiting saves to one matchmaking mode or the other will. Maybe some sort of bonus tied into playing with others -- a wing is more than the sum of its parts, after all, and maybe clients would pay more for a wing of 4 than for four separate commanders doing the same thing? Maybe clients would pay more if they knew their cargo was on a ship that was being escorted or part of a convoy? Basically what I'm proposing is a fix for something I don't believe is broken, but ties into things -- like wings -- that solo players aren't going to get involved in anyway.
But you can switch groups, so... what, have to make a new background sim for every group you're a member of? And have a separate save for each? Does the above recomendations also apply to people in private groups? Why or why not? Seriously, people, Groups is the elephant in the room and complaining about Solo v. Open is utterly useless without taking Groups in all their permutations into account. I'm not against ideas for handling things like the interaction between solo/groups/open with regard to community goals (and faction influence if it ever becomes important in the game) but there are a LOT of factors to take into account that you really can't casually handwave.
But as for the people who just want more targets to shoot at: heck with 'em.
* - Sorry, I can't think of the game's name without hearing it in Strongbad's voice.