I think whether 'balance' was a meaningful objective is pretty well up for debate, too. Sure, if they were aiming for any sort of robust balance, they missed. If they were aiming for balance only among specific mixes of classes on 6-8 encounter days, maybe they hit it. But, if they were just aiming for loose guidelines that DMs could adapt to however they wanted to run their game, while delivering classes and game play that really evoked past editions (that weren't well-balanced at all), and leaving enforced balance to the DM if that's something he wanted to prioritize, then they came 'pretty darn close,' indeed.
5e has been called just a starting place for the game, and balance is a more emergent quality than that, there's little point to designing balance into something when the expectation is that it will be changed, anyway. There's still inevitably things that affect balance, of course, and thus, changes that need to be made advisedly.
The 4e treadmill and 5e bounded accuracy both stretch the 'sweet spot' as far as they go, which is, primarily, in making combats play out similarly at all levels, while retaining some sense of advancement. The treadmill provided advancement mainly in terms of bonuses to d20 rolls, BA restricted those while providing advancement mainly in terms of damage/hps. Even then, though, there are differences as you level. In 4e, characters become a little more complex at 11th level, when they gain new features, and at several points after 20th. In 5e, spell progression is still dramatic enough to change game play as you level and PCs are very fragile at very low level, but the exp chart is tuned to speed play through those less-sweet very low and higher levels, so that's something, anyway.
Obviously, 4e was 'better balanced' and 'expanded the sweet spot' more than 5e did. The former, though, I'd argue, wasn't even a meaningful goal of 5e, and the latter was necessarily compromised to retain more of the feel of the classic game (which includes having a distinct mid-level sweet-spot), said feel being a very important goal of 5e. The result is thus entirely successful, if we make the right assumptions about what they were trying to do.
The lies of the edition war were repeated more than enough to become truth to those who wanted to believe them.
5e's trying to be all D&Ds to all D&Ders. Empowering the DM to not only change/add-to the game but to make rulings to keep his campaign on the rails and w/in his prefered style/theme/tone/etc lets it get just about as close to that unobtainable goal as might be possible. That means that balance is virtually a non-issue from a design standpoint, and one of many things that the DM is going to be supplying, himself.