Why not consider a monthly rate for personal users too? The two biggest complaints against subscriptions is that people are afraid to lose access to C3 if they can't afford the yearly subscription and they are afraid they'll lose access to their game files when they fail to renew their subscription.
A monthly rate addresses both of those concerns. Pay for the months you can afford and if you need access to your game files, pay for just that month to regain access to them. Currently, it's only $99 a year for a personal license, which amounts to about $8.25 a month. That's a really affordable price, but maybe some people might have a hard time coming up with $100 lump sum. Maybe boost that price up a little for the convenience of being able to unsubscribe at any time, and providing a little cushion to Scirra for the times those people aren't subscribed. Maybe $15 month?
I think one of the fears indie developers might have is that if they lose everything by some tragic act, and by not being able to afford to pay for C3, they will also lose any way to make any money back. This kind of solves that in a way that it's not too difficult to scrounge up $15 to regain access back for C3 and finish a game they've been working on. It works for Netflix and other subscription based companies. The reason Photoshop has a $10/month plan is because Adobe got a lot of love from it.
But then again... $99 a year is super cheap already.