Sadly as the general population moves forward and reaches towards level cap, the use for copper or should I say low level materials becomes obsolete. Since they're not used in high level products there is no use for high level players to purchase low level materials to make items or gear. In a sense this restricts the economy as most of the activity for goods will occur from materials that are towards the end game.
Personally, I think materials should be use for all levels of the game because materials like items and gear becomes useless once a player out levels them. If materials are only good for a certain level, then like crafting no one wants to waste time with them as players level at a pace way to fast to even enjoy their materials or products. What would be the point in making items or farming materials when in a matter of a few hours they no longer have a use for the player?
I believe Eve Online uses a good example of how all levels of materials even the basic materials are always used in all products even in the largest ships or weapons. Not to mention Eve Online doesn't have such a huge restriction on ships and gear. Better yet, a player in the most basics ship (T1 ship) can still compete with other players in a more advanced ship if their ship is fitted properly. But in the end, item levels and material levels only segments the economy. While some segments are more active than others, the problem is all segments are relatively small especially when you consider the fact that leveling crafts are very easy.
Although it maybe fun to level crafts because they're easy, in the long run it causes market saturation and since the market is comprise of small segments. The most active segments are going to have a lot of competition driving down prices of goods and even causing markets to crash. Simply put, a high segmented or niche market that is easy to access as a crafter, ultimately makes crafting for profit almost non-existent.