GBB Rifles will continue to improve, the challenge right now is the balance between a low-cost gun and one that will last a long time (better materials).
Once those quirks are worked out you'll have something that will last a long time with proper knowledge and maintenance.
Costs of GBB magazines will go down, as of now GBB Pistol magazines are as low as $9USD a piece for Marui guns.
The only thing that will always pose a problem however is cooldown in cooler climates. But there already is a "solution" to that just the majority of airsofters don't like the idea of having a hose + external tank :P