I know but none of those really make the game easier.
Buy A Theory of Fun for Game Design and read p. 44. Koster explains itfar better than I could dream of. Basically, by always changing patterns, you prevent players from groking how the game works fully and thus prevent them from getting bored.
Another way of looking at the question is asking yourself why new content keeps the game "fresh".
Turbine are designing under some assumptions, yes, because they cannot develop the game while covering all possible scenarios. It would just be unrealistic. So, yes, some changes might have different effects outside of expected gameplay but that's just how life is.