The infusion system is not as good because there are no definite rewards. You can always replace currencies. Instead of just leaving the old currencies in the game you remove the ability to obtain them then exchange them for glimmer. There always has to be a reset in currency with new content as you can't allow people to just buy a whole new set of gear on day 1. The best system Bungie could use is a mix between infusion and EL. Imagine if beating golg and oryx both netted you with a piece of infuse material that increased your gear by 10 points. You'd be guaranteed to get 20 light levels from a raid. You would know this going in. Adding in that EL like item to the raid and IB/ToO would make TTK immensely better.