As I understand it, DOC is only used in lower HP units due to lower emissions standards as defined by the EPA. At current levels DOC is not effective as a stand alone system in those higher HP engines. I had a DOC system in a NH Workmaster 50 that worked well while I had it. Kubota has elected to utilize a Regen system in their smaller HP diesels. No requirements are needed in 25 HP and under.
So to answer your question...
IMHO as long as the EPA keeps current standards as they are I think the emissions systems adopted by different manufacturers will stay as is.