Forcing Mac users to buy adapters and hubs and such goes against Apple's environmental mission.
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Environmental health and safety policy statement
We recognize that by integrating sound environmental, health, and safety management practices into all aspects of our business, we can offer technologically innovative products and services while conserving and enhancing resources for future generations.
That is a narrow view. The huge presumption there is that most folks want to use a USB A slot. Lots of folks would plug next to nothing into their system. 'Extra' USB-A slots that go unused is also a waste.
Apple's keyboard and mouse options .. wireless.
Apple handoff ... wireless data transfer. ( USB-A flash drive 'sneaker net' )
Apple screen casting ... wireless default.
Plug in USB-C phone charging cable ... USB-A is actually a mismatch.
Those who need USB-A buy one. There is no 'conservation' need there because they had the need.
Yes, there are sizable number of folks who plug-in > 2 and < 5 things and have larger stacks of legacy USB-A peripherals... but is that the majority of users? Are most users using it as a 'headless' laptop (which do OK with no USB-A) or as mini-full tower replacement box? Pretty likely this " lots of other legacy stuff" isn't the majority of users.
Folks who have > 2 USB-A device to plug up are going to need a dock of some kind anyway. If needed a dock anyway it doesn't really change much. Still need the dock. If this new chassis is lacking SD card like slot(s) then in similar issue. Needed 2nd Ethernet ... same boat as a deduction from the 'need' class. The 'outer fringe' tower replacement class that Apple is supposedly missing was largely in 'dock' status anyway.
Rack/server deployments ... classic USB-A keyboard and USB-A mouse needed all the time? Probably not. Need to do maintenance physically hooked to the box... plug in a dock with monitor , keyboard , mouse. Do the need .. move on to next with the same dock. Conservation, you only needed one set of ports that you move from box to box. There are many 10's of thousands of mini's deployed this way.
If Apple shrinks the Aluminum chassis to a smaller amount of metal that is a conservation also. The smaller the case the fewer the ports. Taken to the fanatical phase, the 7.7" square footprint of the classic Mini isn't as conservative as it could be. Less fuel burnt moving something around that weights less. etc.