You are also discounting the fact that diesel engines will give many multiples greater service hours/miles than a gasser engine and therefore hold more residual value. For example a diesel truck that is in general good condition still holds good value at 250K+ miles, but most gas engine trucks are either junkyard bound or worth very little after 250K miles.
I'm not really discounting it. Sometimes it's a perception/state of mind thing. For example I have a brother with a 99 Dodge Truck with a Cummins Diesel. He's dumped an endless amount of money in it (the motor) maintenance wise just because it's a Cummins Diesel...Had it been a V-8 he would have lost interest in it 10 years ago and probably scrapped it like you point out. And yeah, he'd probably want 10k for it if he sold it right now, just because it's a "Cummins"
A diesel pickup also costs more upfront than a gasoline one generally. Maybe what, 10k more today? So yeah it's not unreasonable they'd sell for a little more 20 years down the line.
My VW diesels were much more problematic and required a great deal more maintenance than either my Chevy Cruze (Not the world's best car) and way more problematic than any of my Dodge/Jeep gasoline vehicles. The VW TDI's didn't last any longer. And no longer than the same model VW's in gasoline either.
But you know what? A diesel VW sells for more than a gas version. Sometimes ALLOT more. Just because it's a "Diesel".
Of course that's a specific model of something, not a benchmark. Had they been powered by a sweet ISUZU diesel of some sort, who knows, maybe I would still be wrenching on them! LOL
A modern gasoline tractor engine would be just as durable as a diesel, or nearly so. Look at how many old gas tractors are still running.
Probably another factor everyone is overlooking is that diesel was CHEAPER than Gasoline in most places up to around 20 years ago. IF diesel was significantly cheaper today it would definitely be a win/win. (Or maybe a win/win/win)?? LOL I wonder had Diesel been quite a bit higher per gallon than gasoline since lets say, 1930...I bet we'd have seen allot fewer diesel anything around-Including tractors.
But you know, I don't buy a tractor based on what the resale value might be in 20-30 years... I doubt too many others would care either?
I'm just saying it probably won't be a negative thing if a new gas tractor in a good hp class hit the market. Besides that, who knows what the future will hold in 20 years. Diesel may be 3x the price of gasoline, or not available at all!