a solution would be that to get this multiplier you would have to be in open for a long time, say 2 weeks.The usual problems with this sort of idea, which still apply here:
1) I spend almost all my time in Open, and 99.9% of it in systems where no-one else is around anyway. I don't mind getting a 5x payout bonus, of course, but it's not clear that it's "for" anything. In most systems it'll make no difference at all to the underlying question of "are there any other players around anyway?"; conversely in the CG system you already met enough other people to have fun. The number of systems in-between that any change in the percentage of people playing Open would matter to is tiny.
(Unless you regularly find "there's lots of people around at weekends but I don't see enough people during the week" to be true, moving a few more people over into Open will make no difference to your actual experience)
2) If you're encouraging people to min-max for a reward, what you will get in Open is more min-maxers. You may find those significantly less fun to play with than the people already there, who are presumably there because they like meeting other people.
3) Because there's often a substantial gap between "time an action happened" and "time the player was rewarded for it", min-maxers might well do all their stuff in Solo, then leave their PC on and docked in Open overnight to build up the multiplier before cashing in. That's a waste of everyone's time, doesn't get you more actual interaction, and also you might have wanted to use that landing pad. Closing down all those "exploits" makes the idea massively more complex; ignoring them means it doesn't succeed at what you actually want.
it would be something easy to get for the player who is always in open, but difficult for people who try to switch modes to get that advantage.