Most of that effort would be spent writing drivers. Writing a scheduler (how many has Linux had so far, 3?), process manager, virtual memory system, simple file

Most of that effort would be spent writing drivers. Writing a scheduler (how many has Linux had so far, 3?), process manager, virtual memory system, simple filesystem and networking (TCP/IP) stack is not that much work. Supporting most existing hardware, however, takes years. EDIT: for an example, compare linux/drivers/ to linux/kernel/ (core kernel code, including process management and scheduling) and linux/mm/ (memory management) in Linux. The former is huge.