I wouldn’t go far as to say it will never happen, but you’re correct that this is a large lift. Essentially underneath the hood the chromium process model operates differently than how Firefox does which means implementing this is harder and requires more investigation.
This is definitely on our radar and I think there’s interest by a few people to see this get implemented but given its size we’d need to figure out how to prioritize it relative to other work happening.
Without having dug too deeply into this I can think of a few ways where we might be able to create “shadow profiles” and then layer on a differ UI. However, the concept of a profile has impact on a lot of browser preferences which would likely lead to side effects. One example is passwords in the password manager would not be shared across and requiring extra work.
The other idea is if people just want account management containers (e.g. logged into 2 accounts at the same time) we might be able to leverage the ephemeral storage, but then we may end up needing to create separate frame contexts for each in which case might needing to tamper with the origin to avoid other side effects. This usually means there may be implications with the security model around origins and all of the sudden this makes implementing this much harder.
Hopefully that gives some insight about how I’ve been thinking about it and so others can can understand the complexity of why this hasn’t been done yet.