While I would love to see numbers go back up - I also think that a hurdle to that happening is the difficulty inherent of solo ranking with certain classes. Normally, it doesn't affect me much because I play a lot of melee heavy classes. But recently there have been a few not so tanky classes that I've solo ranked with. It's absolutely painful sometimes, not all the time..but sometimes.

Making a change to solo XP would in essence maybe up the # of players -but at the same time might not contribute to more grouping.

I understand it is quite the hurdle, and that there is likely no clear answer.