Water0202020 I don't think they would do that even if it's a good idea on its own.
I'm taking an example: "It's not easy to be a man after travelling to the future". This novel have a steady release rate of 7 chapters per week. Because the releasing chapters 1 per day, if you are at the latest available chapter, the new released chapter won't become a premium one. Since, according to your idea, they should unlock one chapter per day from where you've stopped reading. To illustrate, here is with numbers, let's say you're at the latest chapter available which is 190, the translator publish a new chapter the next day (191), you go to the novel and since you were at 190, the 191 which is premium for everyone else is free for you. So they don't get SS for people who keeps reading.
In the same way, if you say, one free chapter unless it's the latest, you'll create an intent for people to stop reading at chapter before the last one. Since they would have a free chapter everyday just before the latest chapter is released.
So this idea would work as long as the release rate is higher than or equal to 2 chapters per day. But this would also mean that QI would lose 50% of their SS per day (if it's only 2). In other word, they would give one premium chapter for free to readers that are at the latest chapter. It's a costly choice and I don't expect them to do it.
But then you have the mass released chapters exception and here your idea would be something QI might agree to do. Mass released chapter can be a great buffer and some novels have 50 to 100 premium chapters because of periodically mass released chapters and high release rate. If someone has already bought 50 premium chapters from a novel and there are still 20 premium chapters left to catch up, he would either need 50 weeks to be able to read another chapter or would need to buy the other 20 chapters left. In this particular case, people won't wait and would be compelled, at some point, to buy those chapters. And this is really a big problem because here, if you are a premium user, you only can stay premium or stop reading for a very long time.
Two possible solution for QI, they also make mass release for free user time to time reducing the time someone who already bought them have to wait for free chapters. Or they can release a premium chapter for free per day for those people (your idea). In the first case, it would be simple and they can do it now since it doesn't need to be programmed by the devs.
For the latter, it would mean programming a new functionality in the website so it would take time. However, it's better for QI to it this way. Since people would still have to buy the mass released chapters if they want to read ahead and would mean a less costly option.
However, people still give their SS for chapters and still buy them. If you think about it, QI doesn't feel the need to find a way to help people who are stuck in middle of mass released chapters. Even more, they would say that they give free SS every day through Power Stones, Ads and Check-ins and that people in this situation just need to buy their premium chapter with that and they need more, they would need to spend a little more for SS or just read something else.
That's why even if the basic idea is great, QI won't do it unless people move away from premium titles and don't spend any more SS on them.