The cost of that 9N versus what a simple tractor costs today….. and the fact that 9N had paid off it’s owner many decades-ago…. would have to be figured into the equation to compare apples to ORANGE (s).
My 1939 9N did its share of paying-off it’s cost over 18 years of hard mowing for me from 2000-2018 and is now continuing to earn it’s keep with it’s latest owner. (It did burn almost 4 gal-per-hour of gas compared to the 2 or 3 gph of diesel my M4700 uses mowing the same area.)
Most definitely the cost per year of the 9N v the Kubota is so vastly different there’s no way any increase in fuel efficiency could make the Kubota truly less costly overall. I have no expectation the Kubota will be running 60 or 70 years from now. And I agree the 9N was/is a great tractor and an impressive advancement in its time. It punched above its weight for decades. I still use the 9N every time it’s a better fit for the job than the Kubota. Which translates to I visit it occasionally and might get in a load of firewood on the carryall for nostalgic purposes but don’t use it for real work.
The real efficiency of the Kubota with loader, third function, three rear remotes, top/tilt, 4WD, HST, almost twice the PTO hp is in what it will do that the 9N simply is not capable of doing. That’s gotten to be more and more important as people have moved off, died off, aged out such that what used to be several guys with a little tractor is now more one guy who makes up for being one guy with a lot more capable machinery. Still love the 9N and always will. From a standpoint of what it can actually accomplish its not even close to the Kubota.
Oh, and speaking of cost of the 9N, my father bought it for $300 in 1965, rebuilt the engine and hydraulic pump, welded up a list of rather important broken stuff, and ran it hard until probably 10 years ago when the tractor and owner both started slowing down a bit. No telling how many thousands of hours he and I both have on it. It was a great machine. It just isn’t up to the jobs we need it to do.