You seem to be searching for some type of justification for what you are wanting in Classic, but it just isn’t there honestly. They are two separate clients. The games will not link in any way. You will not be logging into retail WoW and choosing a Classic “realm.” You will be logging into a completely separate client (game) for Classic.
Classic is based on Vanilla WoW, the version of the game before any expansions. Not sure why you are misunderstanding this so much. Just because people played the retail version of Vanilla when it was live, myself included, and have items they obtained then, doesn’t mean those items will be there for me in a new client and version of the game as it was years ago. Again, two…separate…games.
Regardless, Blizzard has already said what you are asking for won’t be happening with a few people here in this thread posting their legitimate sources when this was said.