The task sounds enormous
So, two low-scoring vulns could be combined into one big problem. Sure, theoretically, but how do you evaluate just how many low-scoring things can be combined and in what way, before you can rate all of them properly ?
Security is always in hindsight. We know to look out for privilege escalation issues because some hacker one day taught us that it worked. We have a body of knowledge today that is certainly impressive, and it will be one hell of a task to knit all that knowledge together to create a proper rating system, but there is no such thing as automating the risk evaluation - it has to be analyzed by a human. Humans don't know everything, and are rather bad at taking into account hundreds of parameters at once.
It is obvious the CVSS is not very valuable, but crafting a good replacement is going to be a massive headache. And yet, it should definitely be done. Good luck with that, then.