In future when we have enough processing power and memory to store the (almost fully) destructible environment, the graphics artists are going to be the bottleneck.
In current games the artists have to model and paint/photograph textures only for the area player can move and see. With current limited movement, all areas hidden behind corners, walls and other obstacles do not need to be created at all. Also areas only visible from far distance (visible from windows, through a fence, over a blocking object, etc) need only to be modeled with very low polygon and texture detail (often needing only a area specific sky box, as human eyes cannot really determine the perspective on far away scenery that well).
Now imagine you could tear down that fence, break a window and jump out of a building or explode rocks in a blocked path. This all would require the whole game world to be modelled and textured in full detail, including countless of new objects that would not be even visible if the game area would be limited. In a in-door game this problem would be even more severe. The player could break all the walls. Inside structures, pipelines, electric wires, filling materials, etc inside every wall in the game area would need to be modeled and textured in high detail. Also all the (previously unreachable) rooms behind those walls would need to be modeled and textured, and nothing really prevents the player exploding the wall of that new room either, revealing another room, revealing another room, etc, and ultimately getting out of the building to the open terrain. Even without the possibility to get out of the building, this could easily over 10x the work needed by graphics artists (and still the developers would need to limit the amount of destruction). The art creation is very expensive in the current generation titles (often more expensive than programming), and it's becoming more and more expensive every year. Fully destructible environment might sound like a simple feature, but considering the effect it causes to the game project schedule and cost it might never be feasible, unless some kind of automatic content creation system is invented that reduces the artist workload dramatically... and we would need to have automatic testers also (as the testing workload to cover all the possible scenarios would become impossible to handle by human resources only).
Every object breakable inside the game area (bounded by walls and other indestructible obstacles) is a much easier task to complete, and should be achieved in many next generation games.
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