When Apple restricts support on it's Macs to 6-7 years (mid 2012 is required for Mojave btw...) one year is quite a lot IMO. Also, I see no reason not to include the latest chips when Intel releases them except to do more profits on the older chips.
Why would it be more important to have the latest tech in a $999 phone used for Emojis than it is for a $3k computer used for work?
Apple has never been about giving customers the latest and greatest hardware components. Windows machines are there for that, which are essentially parts slapped together with Windows thrown in. I imagine (I would hope) that more time and software engineering is involved to create the relatively smooth and integrated iMac experience, so at some point they probably gather the parts and the specs and produce one generation of iMacs over a period of a year or two. I grew up on Windows PC's, and was always upgrading and tinkering to have the latest gear. However, it is when using my iMacs that I've been the most productive. Not an unreasonable business model, unless you give no value to the total iMac experience.