Do you think they will ever do new archs or anything going back to level 1 sort of fights? The level 40 ones are pretty annoying even if your whole team is maxed out.
I mean the Story Chapters on Normal are easier and the Special Arena stages for the reward units will likely also be manageable without a bunch of lvl 40 units. You usually run a Story mission only once so they will keep getting harder.
Their goal has to be to provide enough content so that every type of player has enough interesting challenges to take on. There isnt much content right now...so if you hit a wall it can get frustrating fast.
Though i dont think there are missions out that are really difficult if you have a team fully maxed. Otherwise i would just wait until you have units to beat the stages - doesnt make sense to waste stamina on story chapters when you could level units in the Tower while its on 1/2 stam.