Now, if you want to debate this in good faith you could suggest that the decreased size of the SoC could easily led itself to a modular approach to Macbooks the same way I suggested it for iMacs. Replacing the SoC via cartridge or slot, akin to a M2 drive would be glorious.
Not a bad idea - but a few likely problems.
First, the cartridge/slot is likely to be proprietary - and in reality (esp. considering that Apple want to make money) you'd probably only get maybe 18 months of upgradeability before the standard changed - possibly for good technical reasons (e.g. the M4 seems to support different TB port configurations c.f. M2/M3 - would the "cartridge" include the USB-C and other ports?) if not just planned obsolescence. So you're looking at a short "upgrade window" during the time where your Mac will still have a pretty high trade-in/resale value. Maybe better than nothing - but it wouldn't be a solution for those of us (say) looking at a 2017 iMac with a display panel that is still ahead of the game lumbered with an obsolete 7-year-old Intel CPU/space heater.
Also - to get enough economy-of-scale to make it affordable
and avoid paying $$$ for an engineer to fit it - this "cartridge" would need to be installable by the average user who could pick up a screwdriver by the right end best of 3 tries. Something like adding RAM to an Intel iMac is probably the upper end of allowable complexity. For starters, that probably means having at least part of the heatsink on the cartridge. With Apple Silicon, that obviously includes RAM and, probably, SSD (does the SSD interface stay the same between SoC generations?)
For a laptop - any such arrangement is going to add bulk and weight. Might not worry us old farts who remember the PowerBook G3 "Wallstreet" as the epitome of portability, but I don't see Apple doing anything that might make the MBP 1mm thicker. Also a reason why any such system is unlikely to last more than 1 generation.
For a desktop - the more I think about it, the more this card/cartridge sounds like a Mac Mini without the case and PSU, at which stage you might as well get one of the various kits that let you mount a Mini on the back of the display. Maybe Apple will have some such scheme associated with the rumoured ATV-sized M4 Mini (esp. if they make it USB-C powered so it could dock to a Studio Display with a single cable).
In terms of "waste" - replace the Mini/Studio computer in your "separates" system with a new model and you still have the old, probably still usable, Mini to re-purpose, re-home, sell or keep as an emergency fallback. Replace a CPU board or cartridge and the old board/cartridge becomes a piece of nonfunctional landfill. Coupled with my point above - i.e. we're talkinga about upgrading 18 month timescales, not 5-year old vintage systems - reselling/re-purposing is the better way to avoid waste.
What is particularly frustrating about my old 2017 iMac is that, if I could have "split it up" or connected multiple computers, I could have used the display as the main display for my Studio
and found uses for the computer part when I occasionally need to run Windows, x86 Linux or old Mac software - and as a still-just-about-viable fallback if my main system failed. Having it in all-in-one form that needs half a desk to itself is
hugely inconvenient. Getting a monitor conversion kit is a possibility (but then the x86 guts and a 1TB SSD go in the trash...)
Pre-iMac I've always found uses for my "last" computer that didn't involve throwing away most of the guts.
