Earlier quoted context omitted.
You're generalising over a whole bunch of devices with different specs. Sure, there was a problem with updates on the 8gb devices. But that was few hardware cycles ago, around 2.3. Now, those devices are around 1% usage, mostly have dead batteries and other components and it's hard to find a new Android device which lacks space for upgrades. What to do? Upgrade the hardware. It's not supported anyway. Modern Android…
I had this problem all the time on a Galaxy S4. I don't remember now whether this was a 16GB or 32GB model; either way, it quickly filled up pretty much entirely by system data and apps. Having an external SD card for everything else didn't help much, and I spent over a hear with phone storage hovering barely above 500MB (if you go below, half of the stuff on the phone refuses to work).
I suspect this is an artifact of using the Linux kernel. As said kernel puts a focus on IO ops, and the EXT file systems spend a whole lot of time looking for contiguous free space before committing a write.
Thus if your app do a bunch of writing (say syncing local data with the cloud) at start, it will start quite slowly on a near full Android device.