Hmm, in a way, I'd say somewhat the opposite could be said, too. When a series has been in play for this long, fans have long since become familiar with all the tricks and inner workings of the systems (even though each has its own little variances). Thus, for the last few years, everyone thinks the games have been easy (though FF X really was), when it's really been our own momentum pushing us through.
Making games harder really makes sense, because then fans get a new set of challenges each time, with the same kind of gameplay. If they don't get harder, the gameplay can get stale. So far, they've offset it by changing up the system each game, but that'll only go so far, I think.