The butterfly was always designed to be the best keyboard they could make that only allowed for .5mm travel, using it on the MBP was definitely a mistake, I would suggest it's even more about not developing a specific thinner MBP keyboard (i.e. the one they finally now have) and component sharing across the lines for economy of scale. Presumably they had in mind that they wanted the 2016 to be thinner, lighter, faster like all previous MBPs were (in itself not inherently a bad goal) - so they wanted the keyboard to be thinner, why go to the trouble of making a new one when they already have the butterfly from the MacBook?
Looks like the MBA might be the one to go ARM (It's big enough it will be easier to re engineer for the magic KB (as you say Butterfly is now pretty toxic)) - they already only offer one (i5) processor option which makes it easier to just offer the single "A13M" or whatever, quite possibly WWDC next year.