Design for performance or heat is different. Would you design an M1 for office work/student with 4+8 cores? No it would be a waste becuase office work does not utilize 8 performance cores. Single tread performance is however dominating in office work but the M1 is more than good enough in that department.
Actually, heat/power consumption (same things, really) is one of the major design constraints on performance for
any CPU/GPU, whether it is intended for a phone or a rackmount server. More cores - or more of any other circuitry - generate more heat, faster switching uses more heat (If I remember my electronics correctly, all other things being equal, heat goes up with the
square of the clock speed...). Having the LPDDR RAM mounted on-package to keep the leads as short as possible is partly about power consumption. Even if Apple produce a Xeon-killer for the next Mac Pro, there will likely be
some trade-off of features vs. power consumption (look at all the expense and effort that went into the Mac Pro cooling system). Dumping the over-complicated Intel architecture makes those power targets a lot more achievable - but I suspect that any future AS chip will feature
some trade-off between performance and the power constraints of the form factor.
As for 8 core "office work/student" machines...
If what you
really want is a machine for word processing, email and spreadsheets, a $500-or-less PC or ChromeBook (or a used 2012 Mac) will do the job with power to spare. But it is 2021 - office workers are running video conferencing software with virtual background filters, home users are editing videos for their youtube channels and using AI/ML algorithms to touch up their holiday snaps, students are mixing music for their band... and the web page you order your office supplies from now features real-time 3D animation (along with enough just-in-time-compiled Javascript to run an Apollo program) unless you jump through hoops to block it.
The only reasons
not to put 8+4 cores in a general-use machine are (a) cost and (b) form factor (i.e. size and thermal/power limits)... and if you ignore (b) then economies-of-scale
could mean that it was cheaper to have a single one-size-fits-all processor.
But for many other people, myself included, who live in pretty much a wireless and cloud-based world for work and leisure, there are few instances where we need lots of additional I/O. And when we do, the versatility of USB-C/Thunderbolt comes into play.
Often, the problem is not the machines that Apple make, but the machines that Apple
don't make. There's no question that on plenty of peoples' 24" iMacs, even those two solitary TB3 ports will do nothing but gather dust. OTOH, some of us have a metric shedload of USB 3 or even USB 2 devices, and third party external DisplayPort or HDMI displays for which USB-C just means a more expensive cable that uses up a valuable data I/O ports on an unrelated function. Apple don't make a machine for us (although, the Intel 5k iMac and Mini aren't too bad and didn't
lose ports c.f. their TB2 predecessors).
(...and yes, there are dirt-cheap USB 2/3hubs but plenty of USB-3 & 2 - such as audio interfaces - are best plugged into top-level ports rather than contending with other devices for bandwidth and latency).
This is all a bit moot with the $1500, 4-port 24" iMac, though, - which most likely maxes out the i/o capabilities of the M1 - so at the moment, the reason Apple don't make an Apple Silicon desktop with more ports could be, simply, that they haven't released the 5k iMac and high-end Mini replacements yet. It will be disappointing if the 5k replacement doesn't have at least 2xTB3 (with dedicated controllers) + 4 x USB-C (flap, oink on the DisplayPort). It's a desktop, so I can live with a few USB-C-to-A dongles (unlike a laptop) - and if you
do get USB4 hubs to unlock the total I/O bandwidth it's not too shabby.
What is weird is the $1300 2-port iMac (...which comes to $1379 if you add the Ethernet and Touch ID options - both available as options on the base model, while based on Air prices, Apple seem to value the 8th GPU core at $49...) - I just can't get my head around why it is worth Apple's while to make
two physically different versions of the iMac (which will create extra logistical costs) to justify a $70 price difference... The options prove that both models can support ethernet and TouchID with the right peripherals so they're clearly identical but for the missing ports and 7-core chip. If you don't need the extra USB-3 ports - they won't hurt. If you don't want USB ports because security - hello, 2 x TB3 with all the risks of USB plus any extra risks of Thunderbolt...)
Either they cynically decided that the prices
had to be $1299 and $1499 and the missing ports are
purely to stop you upgrading the $1299 model to the same spec as the $1499 one - or, maybe, the 7-core-GPU version of the M1 also has reduced I/O...?
The other slightly odd thing is why the Ethernet (i.e. alternative power brick) isn't an optional extra across the board? If there's one thing that people either need or don't want to pay for, it's Ethernet.
