Unified memory does have benefits in speed, so at least on the memory front there is a benefit to having it soldered down now. But that benefit was only realized with Apple Silicon, and not with the intel chips soldered down before. I think a good compromise now would be to add in additional physical slots in addition to the unified memory that add to the overall capacity, but are only used when unified becomes full. This would still allow speed benefit over SSD storage, and help extend the useful life of an older machine somewhat. Those slots could come populated or empty in machines that have the space for them. MacBook Pro’s and all desktops should have the option, but it makes sense not to have it in the MacBook Air as that really is making the size trade off. Upgradable storage though still has no excuse. Every mac should have an M.2 slot in addition to any soldered storage, and that slot should be able to run the OS just the same if needed.
It would be nice, but add costs and complexity for something most users would never use. Companies make cost/feature tradeoffs all the time.