It’s not so much an envelope as in a case but more the core module that cannot be upgraded that other components atache too.
The range of attachables is limited, by the housing/envelope and the SoC on the core module let alone all the other detail of the module. So a new core module is developed.
The copre module is a) the most demading in terms of manufacturing footprint, subsequently the most expensive. So it makes sens eonmce that has been manufactured/designed to design the modules and case ‘envelope’ to fit the core.
So sensibly the core will not be made to fit an old envelope.