WotC is bad at estimating balance on both sides of the screen. I'm not sure pointing to their charts on how tough an adventuring day "should" be is a great argument to be making. The default assumptions of the game (tank, rogue, healbot, blaster) probably hold up well enough against the typical beatstick monsters, but if either side starts building/playing a bit more optimally, or even just a bit weirder, things get thrown off.