Torque is a non-issue. If the gas engine doesn't have enough torque, it's not the right (gas) engine for the application. Engine longevity is a non-issue. As mentioned above, a significant number of 80+ year old gas tractors still run. I also wonder if the fuel economy advantage still exists (or has been SIGNIFICANTLY reduced)? I know in U.S. diesel cars and trucks, the emissions regulations have been decimating the fuel economy of diesels - sure they have better emissions but they now use 20% more fuel. Gas engines are getting more fuel efficient (especially in this 20-50 HP space) while diesels are getting less efficient.
Non-emissions diesel VS carbureted gas = Diesel is clear winner.
Emissions-compliant diesel VS Closed loop EFI gas engine = I don't know, I haven't seen a real comparison.
I'll open a Crowd-Funding page so everyone here can chip in to find out. I'll get a Deere X730 Gas (then have to re-power it with a closed-loop EFI engine) and X750 Diesel. I will alternate which one I use to mow every week this summer and keep detailed fuel consumption logs.
I'll have to disagree on torque not being an issue. Torque level at an rpm is the horsepower figure; the equation to calculate it: HP=Torque (lb*ft) * RPM / 5252. I'll agree that gasoline engines can certainly be designed to generate better levels of torque, but typically are not, because the top rpm figures suffer, which in turn limits peak power production.
Diesels have 5 characteristics that, on average, tend to make them torquier at low rpm than similar gas engines, those being higher compression ratios, higher flame propagation rate, longer stroke length, forced induction, and fuel energy content. Stroke length is a pure design feature that can be added to gas engines for similar results, as can forced induction. As far as compression ratio and burn rate, direct injection gas engines come closer to diesels, but a typical port or throttle body injection gasser like you'd find at the top end of the small engine market today isn't close. Diesel fuel also contains about 15% more energy per unit volume than gasoline.
Taken together, those characteristics provide great torque at low rpm, which results in more power being available at low engine speeds. For non-PTO tasks, this allows one to run the engine slower for the same result, which tends to increase fuel economy. You can get similar results to diesel physics with a direct injected gas engine, but then you've lost the cost advantage, and still have the 15% energy density issue. It's looking like the next generation of DI gas engines will need emissions traps for particulates that are similar to the ones used by diesels now, which will push the fuel economy pendulum back toward diesels.
Old gas tractors had very well built engines, which consequently last a long time. Farmers (at least the ones I know) are, by and large, pragmatists; they pick the option that provides the best value when it comes to long term investments. My father in law is one of them; he still milks about 70 head. The bottom line is that, from a life cycle cost standpoint, including purchase price, maintenance, and fuel, gas tractors cost more to own over the long haul. Now, the ag market is a lot different than the SCUT market, to be sure, and this analysis may not be valid for SCUTS.
Gas engine longevity in modern engines is an issue due to how they're built. If you build them to last, like the old tractors were, they cost like a diesel. Once you get to that point, you get better winter starting, and....that's about it.
I'm on board with your general reasoning for having gas, as I made the same choice myself. But, per what I outlined above, I also understand the decision by manufacturers to not put gassers into tractors anymore, as there isn't much market for it.