> It was originally done based on an incorrect assumption

there were a few motivations originally
- impls may need mutable state
- impls may need exclusive access to some underlying thing)
   - out of these, "exclusive access to an i2c/spi bus for a GPIO expander" turned out to be an incorrect assumption, yes, because these likely have multiple pins so RefCell is needed anyway. The rest of motivations are still valid IMO.