A small, simple and clean suggestion but really helpful. Would show users exactly how many new posts they have to read, users short on time can decide to read longer threads later and this can have positive impacts on engagement (lesser active threads will also draw attention).
We also don't count or cache that level of detail when it comes to read marking.
We know when a thread has unread posts, but we don't actually know how many. The Unread Posts add-on has proved that although this can be counted, it can be murder to do it in a way that works in a performant way. You then have to implement various caching mechanisms but will likely then leave you with situations where the information could be out of date.
So as much as it's a nice idea, it's certainly not a simple one.