I can only speak for myself here but the way I'm playing the game (and games) I want to be on my own and still be able to do a bit of everything. After the skill change when you introduced the skill decay all that changed for me. The only way around the problem was to buy 2 more accounts to spread the skills on three characters.

Considering the way I'm playing games (sporadically and casually) I would enable all three of my characters from time to time if the sub fee was lower (5-10 dollars per month), but not for 15 per account per month, that is more than Im willing to spend on my casual play style

Not sure that helps you when it comes to decide where to go with the pay model, but I guess catering to the Steam community is hard. Most will want the game for free and then have you paying them for playing it...