The Mac Studio's SSD storage is not soldered in place and could be upgradeable, according to a teardown performed by YouTube channel Max Tech.

In a detailed teardown video of the new Mac Studio, Max Tech revealed that the Mac Studio's SSD storage is located in two slots, and is relatively easy to remove or replace since it is not soldered down. The individual SSDs can be swapped between the two slots and appear to be modular.
On its website, Apple claims that the Mac Studio's SSD storage is "not user accessible" and encourages users to configure the device with enough storage at the point of purchase. This is likely because the internals of the Mac Studio remain difficult to access from the outside. The rubber ring on the base of the machine has to be removed to reveal four screws that provide a way to open the device, but once inside, the SSD storage is fairly easy to access and remove.
Max Tech suggested that Apple could offer SSD storage upgrades at a later date, similar to how it introduced a kit to upgrade the Mac Pro's storage in mid-2020, but the easy access could be intended for use in repairs only, enabling repair technicians to remove and replace SSDs more conveniently.
The SSD storage in Apple's other custom silicon-based Macs, such as the MacBook Pro, MacBook Air, and Mac mini, is soldered in place and cannot be easily upgraded, but some skilled technicians have attempted to do so.
Top Rated Comments
But really
It’s just the way it should have always been.
cpu is not much of a bottleneck unless you are running high intensive tasks all the time, a base i3 4 core from years ago is still relevant today for mundane task, this isn't the same as previous core 2 duo or bulldozer generation.
Apple charges $400 for 1tb SSD vs $150 out of pocket.
If you can afford the apple tax, great! But I reckon cost is why a lot of people prefer to upgrade their machines themselves.
Saying “just get 4tb now” for the insane cost apple asks, is quite the statement.