If you think back to the early 70's when the catalytic converters came out and all the problems they had for a while. The new emissions on the diesels are getting better and will continue to improve.
A gas engine will never meet the power and reliably of a diesel engine. As a diesel engine gets loaded up the harder it hits and the more power it creates where a gas engine just bog out and dies. There is a reason why people went to diesel engines in heavy equipment. To use the same engine that a car uses in a tractor the valve timing will need to be adjusted as to make the engine have the torque needed in farm equipment.
If California (where most of the rules are made) would break off the main land and fall into the ocean we would be so much better not having to meet all the stupid ideas of people who have never even seen a tractor.
With new sources of gas like CNG, etc for over the road trucks, buses, etc gas seems to be making a come back.
EPA does not like dirty engines and may kill the diesel's edge in the end.
If diesel fuel continues to cost 25% more than gas and diesel engines continue to cost 200%+ more than gas engines then gas ownership/usage may make to the most financial sense.
Gas vs Diesel today is more of an 'emotional' issue than anything else in the typical CUT usage pattern.
As a kid in the spring we could put 100 hours some weeks on a 30-40 HP engine and it would last years running with its ears pinned back. Today a lot of us do not put 100 hours a year on a tractor.
In PU trucks in can get even worse in a practical sense. Some do need diesels for sure but not when they are only driven empty as a daily driver.
Diesels will be around for a long time but for mowing three acres 20 times a year it may not make $$$ sense in many cases.
My old Farmall A made the same power as the Kubota B8200 I had at the time. Same HP, very similar torque curve. The Farmall was a 1941...
Big different, is the the Farmall was 113ci, while the Kubota was 59ci. And, the Farmall used more fuel. But, it had plenty of grunt...