13-episode seasons or less are increasingly the norm for hourlong dramas, since serialized season arcs work better with fewer episodes and less padding. Heck, people keep criticizing the 13-episode Netflix Marvel shows for being too padded out in the middle, saying they'd be better off at 8 to 10 episodes. And AoS has already broken its past two seasons into 2 or 3 distinct story arcs; doing a shorter season overall seems the logical next step. I see no reason to interpret it as a negative or a vote of no confidence in the show, when it's exactly the sort of thing that more and more shows are intentionally embracing.