There is very little to be gained by writing a new operating system if and only if you want your operating system to be Unix-like in the services it provides. From the 1960s until now, there has been little desire to have an OS provide services beyond what unix affords. Every operating system that you ever heard of, and many that you didn't provide about the same set: they manage machine resources, divide them fairly across multiple applications, provide relatively safe access to device hardware. So Windows NT/XP/7/8 are unix-like, as is OS/X at its kernel, VMS provides the same sort of services, so do OS/360 through z/VM for mainframes.
There are good arguments which say that's all an operating system for a conventional von-Neumann machine should provide. Note well that the kernel is not: the filesystem, nor the windowing system, nor the web server, nor the web browser; it is not the compilers or the editors or shells or applications. Torvalds built on the elegance of the Unix system design which itself was a distillation of the fundamental services of operating systems of the era.
As it happens, the Unix abstraction as envisioned by Ken Thompson and Dennis Ritchie scales beautifully from the smallest Arduino to the largest distributed system. Although Torvalds and crew have been steadily optimizing the kernel and incorporating new hardware features as manufacturers invent them, the operating system is not the bottleneck in systems today. Indeed, the OS is so lightweight that many systems can run multiple instances of an OS along with a virtualizer (which is kind of an OS sub-basement) and the processor spends much time waiting on the very slow memory or the glacially slow disks. Even if the kernel could be made twice as efficient, it might be hard for a person to perceive or even measure in real usage.
Given that there is an Open OS which is well written, efficient and very portable, there's little cause to bother doing it again. But recall my caveat at the top: if and only if you want an OS to provide these types of services. There have been esoteric operating systems which have deviated slightly from unix-like capabilities, they never saw much exposure. There may be call for a wildly new OS type should something some fundamental change in computer architecture calls for it, but if there is one, I've not read about it.
I can imagine it though, if we were to develop a human-brain style computing device, Unix would probably not well serve the needs of the hardware. Would there even be an entity called an OS? Dunno.
[There are many things I could have linked which I didn't (e.g. von-Neumann architecture) because if you don't know what they mean, they'll be readily findable and there is merit in not having me guide you.]