KoraL Yes, I think something along the lines you suggest would work. WebNovel needs to do something for the already completed novels or ongoing novels with a thousand or more premium chapters. People just aren't going to commit the hundreds of dollars necessary for reading them.
I think the current WebNovel scheme works well for keeping up with ongoing chapters. With the current membership plus $5-$10 top-ups/month I can keep up with 4 active novels (2-3 chapters per day each). I'm willing to pay that premium for works in progress.
KoraL, have you looked at wuxiaworld's VIP program? They are one of the few other non-pirate sites out there. I subscribed to their Gold-level VIP program last week, which enables me to read chapters from 3 completed novels of my choice (I can choose novels the first of every month, either continue on with the current ones or switch) for $10/month. (Silver level is read chapters from one completed novel per month). They have separate mechanisms for funding their ongoing and completed novels. I think something similar would work here as well.