You are correct, diesels are cheaper to own per mile driven. I'm not arguing that point at all, my point was that how much you drive and how often/heavy you are towing dictate how long it will take to pay back that initial extra $10k investment. I didn't do a good job of articulating that, and I'll probably screw it up again here, but I'll make another attempt at it. The extra stuff I mentioned (DEF fluid, Oil changes, etc.) takes away from the fuel economy savings making the payback period even longer.
Let's take one example on fuel economy alone. Obviously there are a lot of factors that change based on where you live, how you drive, etc. but to simplify it I'm just running it a few quick ways as an example. Do your own math for your situation and things may shake out differently. Again, I'm not referring to 1/2 ton trucks here, to me that's a different deal and if you can get by with a 1/2 ton, that's the route I'd go.
Current National Avg. Gas Price: $3.19/Gal
Current National Avg. Diesel Price: $3.29/Gal
Let's take my average 12 mpg gas truck and a 20 mpg diesel (plug in your own number if I'm off here)
If I drive 5,000 miles per year it costs $1,329/year in gas and $822/year in diesel
10,000 miles it's $2,658 vs. $1645
30,000 miles it's $7,975 vs. $4,935
The gap between the two obviously grows if you do more heavy towing, etc.
I fit into the 10,000 miles a year category so it would take me nearly 10 years to get my money back from spending an extra 10K up front on the diesel, and that's before you consider the more expensive oil changes (every 5K for me diesel or gas), DEF fluid, etc. Sure, it may not be a ton extra, but it does effect the payback period.
This also doesn't take into account the time value of money. If I take that $10k and invest it elsewhere I'm way better off earning a return on my money over that time period, and if you have a loan, you're paying interest on that $10k making the payback period even longer.
Sure, the resale value can come into play in certain situations but for me, vehicles are depreciating assets so I consider them an expense, not an investment and the resale value is a very secondary consideration. Your situation may be different swapping vehicles out every year and that may sway your decision, for me personally it doesn't.
Again, my point is to do the math for yourself and see how it shakes out.
As another point, my father in law owns a diesel F-350 that gets less than 5,000 miles a year put on it. He uses it to pull his fifth wheel trailer camping and that's about it. From my scenario above, he'll likely never get his money back vs. if he'd bought a gas truck. That doesn't mean he should have bought the gas truck, he likes how the diesel tows so for him it's worth the extra money. Do what you want, just don't assume it's right for everyone because it's right for you in your situation.