There is more money in boxing for the fighters, and there is a deeper talent pool in boxing....those are facts. Those facts lead to the conclusion that it is more difficult to succeed in boxing than it is in mma. sure, in mma you do all disciplines of fighting and whatnot. but the talent pool is shallow in comparison, the level of competition is not as high - and those are the key elements that determine the difficulty of competing in a sport. mma is simply not developed enough to be more difficult to compete in than boxing right now.
To say mma is tougher JUST because it is more varied is flawed. that's like saying being a jack of all trades is more difficult than being a master of one - that's not necessarily the case. The fact that high school teachers, WWE wrestlers, engineers can just leave their occupation and compete in mma at the highest level within a just a few years (after their physical prime btw) is TELLING. Training in boxing is more difficult because boxing isnt a natural thing. When someone throws a punch, based purely on instinct people put their hands up, they try to walk or move out of distance, they try to grab the hands of the person throwing the punch. What separates one great boxer from another is often intangible things that you cant train or teach like heart, mettle, chin. The action takes place in a very confined space with a limited number of techniques one can use in the ring. Great boxers are trained to do the unnatural at an instinctive level. Watch someone like Floyd slip and counter a punch - that isnt natural. To stand ones ground and understand the distance and timing to not get hit while throwing a counter can only come with years of repetition to the point where it becomes instinctual.
MMA as a sport in general, to contrast with boxing, is more often a sport about exploiting the weaknesses of your opponents game. A fighter with a background in wrestling often wont look to engage in standup with a guy who has a background in kickboxing. He takes the fight to the ground and tries to exploit the perceived weakness of his opponent. Rarely do intangibles like heart and mettle come into play because rarely will you find in MMA a fight that is a war of attrition. Often fans see very early in fights who will win and who will lose because they can see where a fight will go and who holds a distinct advantage over who in a specific aspect of the game. Doesnt mean thats always the case since the small gloves and quick subs can change all that, but more often than not people can see who will win a fight based on these advantages going in. In the end these are facts, truth, and reality. You can't ignore what is really real and what isn't. Fans of mmma need to worry about there own sport and stop hating on a sport that has already stamped it's mark in history and popularity around the work has been established.