Further, when I buy a new one, I’m now incentivized to over-provision it based on my current needs by that same logic.
OWC has an entire business around this (for older Macs): https://www.owc.com/
Photos and videos get larger each year with larger sensors, so it can be hard to predict future usage if you take a lot of those.
I used OWC parts to make a 16 TB m2 SSD array that connects over Thunderbolt. It's fast enough to edit 8K footage, just like the internal disk. Look for the 4-bay Thunderbolt enclosure on Amazon. I did add extra cooling (heatsinks on the modules, and a bigger fan).
Total cost was about $2000.
USB4 can hit those speeds, but then you’re in dongle town.
Back in the day when this was possible (iBooks, Powerbooks, early-model MacBooks), I'd say that a large percentage of Mac laptops eventually did get upgraded. I certainly upgraded 100% of the Macs I owned and also did many for friends and family. Some models made upgrades quite easy: the RAM slots, especially, were often accessible without special tools. It was common to buy the base model Mac with the fastest CPU, then install your own RAM modules and big HDD/SSD to save money. Swapping HDDs out for SSDs was also, of course, a huge performance upgrade for a while.
Even non-technical users who wouldn't upgrade their Macs on their own would often trade them in to dealers/resellers who would refurbish and upgrade them for resale.
It was even easy to do it, no need to take it all apart. There was a lid behind the battery.
Remember that for people who used computers in that period, opening a computer to replace a component was a completely normal operation.
That said, I do think upgradable laptops are important as a resistance force against constant upgrades and planned obsolescence; if you could hypothetically add 2x32GB DDR4-2100 to a decade old ThinkPad and run stolen Apple Intelligence LLM just fine, the humanity wouldn't need $5k worth of labor wasted on one laptop per person per year.
You're saying few people would buy the 256GB model and pop in a fast 2TB M.2 SSD for 103€ if they could?