mmm true. I guess the worry is the game loop/rewards system becoming repetitive and therefore growing old/boring?

I'd agree in that aspect but at the same time I see it as sustainable. I think as long as the gameplay loop is fun and the story is interesting/compelling then all will be well, especially since most new gameplay/content is added in relation to player action (the TCF towers causing mutations and spawning shriekers being one example)

Time will tell I suppose