I agree, but that's because you're starting with an existing kernel you're trying to improve. If you were a startup-sized team with no kernel programming experience and you had to write an entire OS from scratch I think you'd be pretty likely to end up after five years with the software equivalent of the non-competitive microprocessor designs the grandparent describes...

[Note the caveats: obviously history provides us with examples like Linux of OSes from scratch which did get from zero to competitive in five years but that had rather more people working on it.]