What would it cost to develop from scratch? How many engineers do you need? How many lawyers?
Anyway, why do you think that's a problem you need to think of?
You could probably spend a trillion dollars reinventing the kernel and still lose out to Linux in a world where the internet still exists. If you insist on a commercialized outcome then no amount will really be enough.
They're all basically the same thing.
Heck Android and iOS are basically the same thing.
Would a new commercially viable OS be able to offer anything that current ones don't? If not, it's not really commercially viable.