You are just making stuff up. The game in its "intentional" design had no trade initially, yeah you had to make non boosted goods as well. Guilds weren't part of the game. Friends weren't part of the game. When trade did come it was only with neighbors. Who knows why they do what they do and why they don't, they rarely ever say. And you know it's not like it's such a groundbreaking radical new idea...The idea of only having 2 of the 5 goods available from each age means that a big part of the strategy in the game involves solving the problem of limited supplies of the other goods. This, combined with the market limited to guild, friends and neighbors puts a premium on finding a good guild and picking good friends. A global market would take that away from the game. It wouldn't matter who your friends were or what guild you were in as far as finding the goods you need. It's not just that the boosted goods mechanic has been part of the game from the beginning, it is that it is a fundamental and intentional part of the game's design.
The other considerations when talking about a global marketplace are the sheer size of such a thing. There are multiple thousands of players on each world. Even if it could be done by the developers without seriously bogging down the game, who would want to wade through a few thousand pages of posted trades? It would simply overwhelm new players. It's just a bad idea that changes a fundmental challenge of the game.
No, it's more like this. I purposely chose to not have a cat because of how I want to live my life. My life is designed to have no room for a cat in it. Therefore, I will not get a cat.
Bottom line: Might there be some benefits to having a global market? Yes. Might there be issues with the game in implementing a global market? Very possible. Would it change a fundamental part of the game? Yes. As Algona would ask, "Is it good for the game?" Very debatable, but my answer is no.