X5 was the poetic and awesome ending. Then Capcom wanted money.
This. The Nightmare system in X6 made things almost impossible at times. In one of the final levels, if you don't have a certain part equipped, X can't complete the level. You get that part, and others like it, from rescuing reploids in the stages. The Nightmare enemies can kill those reploids and lock you out from getting those parts forever. Some of them are almost impossible to get and survive.