There will be always a fair window for boxing, MMA drag a certain audience, Boxing another one and I am absolutely certain that there is places for both: Some peoples just don't like the idea of a fighting going on the floor. In MMA, it is more than possible and even with the rules that you cannot spend more than some amount of time on the floor, grapples are impossible to avoid and the mat is more than likely to get in action at sime point. On the other side, some peoples think that boxing is too limited MMA as it is obviously not as complete, it's all a matter of taste.
PErsonally, I think that MMA is not killing boxing but will help it's revival: MMA forced boxing to make a few adjustments, especially in raising the general stake of the cards and forcing the display of a better show in general, it did force boxing to rethink itself and the way it functions, it will force promoters to find more often the most exciting fight possible rather than the less risky, it gives a new philosophy about cutmen/cornermen/promotion etc, a new way of thinking and as far as I am concerned, there is nothing better than something that forces yourself to get redefined and to hit a little "conscience crisis". When you're on the top of the mountain all alone, you do not feel the urge to push your limits or you don't see a reason to improve anything. When some competitors kick in and could steal your crown, than things gett different and you want to find other possiblities to improve, to not let them take you down.
Taking everything for granted is bad, as much for individuals as it is for sports, MMA provided Boxing with an excellent antitote to force it to rethink itself and to put in questions some mode of functionning, which was imperative to keep its interest up. Boxing was starting to act like a selfish mother, not caring too much because "fans" would buy anyway. Now it's different, there is an alternative and mother boxing had to redefine the rules in order to keep its audiance.
IN the end, with the MMA as competitors, I think it's a win-win situation for everybody.