It won't happen because your system would give freedom to the gamer and Bungie isn't interested in gamer freedom, they are interested in control. They want to control how much and when you can do or gain anything. They see their system as a way of guaranteeing you will keep playing for a certain number of hours per week. They don't want to risk giving the gamers control over progression as they probably think it will make people upgade easier and stop playing sooner. I think a trading system would be more fun as it means you can concentrate on doing the things in the game you like and trade with others to progress