Read through the linked conversation, and it matches pretty closely with what you've suggested. There were concerns about whether making the IDs public could cause security issues, but the concerns were very minor, so in all likelihood, there's a good chance the IDs will be public, probably in a user's profile URL.
Whether or not username changes will always be open will be up to Pokemaster, but with an ID-based system, I see no reason not to keep them open indefinitely.
However, the conversation discussing the update saw activity in 2015, 2017, and 2019, with nothing since, so it's not likely we'll get an update like this anytime soon :\