I would agree with Craig on this, medium to long term as with most businesses your sales will level off. If you don’t have some kind of recurring revenue stream you won’t have the cash flow to continue producing what is a great solid product.
Personally I like the unlimited license model, its easier for me to manage and gives me scope to grow my business but I would gladly pay an annual renewal fee for that luxury.
What about a White Label license for developers that hides things like the Support links and license box, with an annual subscription fee.
Selection of child themes would be cool too.