In a logical world, that would be the case. But, if there is one thing I know about government, they ain't logical. Kubota and others may come up with different systems and there may be some different EPA approved ways of doing things, but the Tier IV rules will stay. (I so hope I'm wrong.) I work for government (state) and I have been part of rule changes and I have basically begged the powers that be to not make certain rules changes due to the consequences that I KNOW will happen. What do they do, adopt them anyway, then comment later how there are some issues with the rule changes. Government is a weird entity, and it is rarely about doing anything with common sense.
I bought my L2501 just because I did not want the potential for DPF problems down the road. I would have jumped all over a L3901 if it did not have the DPF. I was totally ignorant of the Tier IV rules until I started tractor shopping. I came so close to buying a L3940 years ago. Oh, how I wished I did, but my career/finances was a little bit in question at the time.
I also hoped Trump's administration might help in rolling the rules back, and I actually did a bit of searching on that. But, as I already know, that wasn't going to happen, at least not anytime soon.
I would like to know how many people like me steered away from a bigger machine just because of the DPF systems. I bet quite a few.