Their older hardware is good enough (and that's the problem)
21" model with a decent second monitor would be a more than adequate set up for me.
I have one of the 2016 27-inchers and it's been a great value machine, still blazingly fast and with the 5k Retina display. But that's the problem: that machine is more than good enough and there's not much tempting me to upgrade. In the same way I have a 2015 Macbook Air which is still - IMO - better than the new models.
Basically I'm not seeing anything from Apple that's tempting me to upgrade. Even if these machines are better than my current setup, they are not *that* much better that I'd bother for a considerable time, at which point they'd probably be available second hand. Have we reached a point where hardware several years old is good enough for most people? See also: phones.