I think Apple is carrying through their business model with all their other systems. T
hey want the user to replace the machine rather than extend the life. I think Apple’s approach towards modularity is with its external connectors such that any Thunderbolt device can be moved between any of their other computers. In fact the latter really is another way to encourage the former as once the update an interface on one system it “encourages” the user to upgrade all their other systems to have the same interface.
In addition there seems to be a decline in “speed bump” updates models.
Mac Pro late 2013
Mac Mini late 2014
iMac late 2015
MacBook Air early 2015
Only the MacBookPro seems regular and that involved TB3 and Touch bar between 2015 and 2016.
So it does seem updates are more likely with interface changes.
Then there’s the odd MacBook 2010 to early 2015 and then early 2016 (and nothing yet for early 2017).