External storage is such a janky solution that requires users to file manage beyond their computer, that's not why I have a computer. Just get me a big coat and I'll dangle my drives on the inside, let's hope they do not fail or get lost.What's that benefit? It's not obvious to me. All I can think of is to prevent people from buying much more memory than their passively cooled thermal-throttled cpu can handle.
Anyhow there is no big fundamental difference between short-term memory and long-term storage. Soldered versus upgradeable has largely the same benefits and detriments in both cases.
Or you could empty your storage on an external ssd drive? Those will get cheaper later in the computers life too.
The usability doesn't really shrink with storage size, but with storage speed. You can't add a faster system bus later on. Or a faster USB port for external storage either.
SATA 3 had a speed limit of 600MB/s. Unlucky me I had a unibody MBP from 2010 with SATA 2. While you could easily upgrade the size of storage, storage speed sucked even with an SSD.
I bet the usable lifetime of Macs has only increased with solder and glue. As for the users ability to avoid Apple's crazy upgrade prices, that's another story.
A lot of the rationale from people here seems to come down to: "why aren't you giving Apple all your money? Just stop questioning and give them all your money."