Relhak They tried it, and it failed.
Unlimited reading for $25/month. Considerably more than your suggestion. It was still too popular. It lasted about 4 months.
Income for the top authors went down by as much as 50%. Since the books by the top authors are the ones that bring in the vast majority of the money, WebNovel has to keep the top authors happy.
Why the income decrease? WebNovel has never said, but I assume I'm fairly typical of those readers who actually spend $20-$30/month here. I subscribed as soon as possible. I was still reading my top authors with their continuing books publishing a new chapter a day, but I was also reading complete 1000 chapters novels of lesser authors. My money was going to completed books (which very little money goes to now) instead of the ongoing chapters. So instead of the top authors getting half of my money a month, they were getting maybe 5% of my money.
Your suggestion would be very reasonable if WebNovel was making money off of completed novels (like Kindle). But WebNovel is geared towards making money off of books currently being written/translated.
WebNovel still should find some way to monetize their completed books. Their discounted weekend book program is an attempt, but not a very good one. I personally would like them to add the ability to read one or two novels of my choice each month (unlimited chapters) to their current membership setup and possibly take away the bonus coins for daily login (making almost all coins paid).