Despite being easy to remove, the Mac Studio's storage is not currently user-upgradeable due to a software block.

mac studio ssdImage via Max Tech

The storage of the Mac Studio could be upgraded since it is not soldered in. There are two internal SSD slots in the Mac Studio, and they can be swapped between the connectors.

In a video on his channel, Miani tested if the storage is upgradeable in practice. Miani put the Mac's status into doubt when he wiped the SSD of the machine, removed it, and put it into an empty slot.

It is possible that Apple is preventing users from upgrading their storage themselves by preventing it from being bootnable. Apple encourages users to have enough storage at the point of purchase, as it claims that the Mac Studio is not user accessible.

It seems that the purpose of the easily replaceable storage is to aid repairs performed by authorized technicians, who likely will have software tools that allow them to boot from different internal storage. Apple could allow users to upgrade their own storage in the future via an update since the prevention of user-upgradeability appears to be due to a software block.

It was not possible for users to manually upgrade the storage of the Mac Pro, but Apple decided to offer a kit to do so in 2020.