Unfortunately, there isn't a one size fits all solution. Plugins generating leads for consulting or custom development, support fees, donations are all definitely viable solutions, but won't necessarily apply in every case. At the end of the day, a developer may need to realize that writing WordPress plugins just isn't going to be what pays their salary. I could name many large (larger than any WordPress plugin) open source products (that are free of charge) which don't generate a significant amount of revenue for their developer.