AD&D doesn't have balanced encounter building rules. It is completely left to the DM (or whatever tables the DM decides to use). So magic items don't "break" anything, because there's nothing to break.
If we don't have encounter-building (or at least adventure-building) rules in 5E that are at least as solid and robust as 4E's, then 5E will have utterly failed at its promise to unite all the editions. Without 1) assuming a magic item progression by level, 2) incorporating a party's magic items into encounter building, or 3) making magic items pure fluff, I don't see how 5E can possibly succeed at the goals that have been stated for it.
I'm not sure what you mean by "solid and robust" here. If you mean "as restrictive and precise" it will fail in the other direction. I think things like bounded accuracy are indications that they are trying to move in a more forgiving direction. That is to say, removing some of the impetus to make sure you must finely tune your encounters or the whole thing breaks.
Also, I agree with your assessment of AD&D lacking something to break...it was a good thing and needs to be resurrected.
On the bigger point of uniting the editions...I don't think that 5e has to
be any edition to attract players of that edition. So, what is the purpose of 4e's encounter budgets, etc.? Is it that method of encounter design or its results that are important to the game\experience? Personally, I've been pleasantly surprised with the strange things people can accomplish with game mechanics. I've played several OSR games that use newer mechanics to produce very fun old-school feel experiences. I wouldn't be surprised if the new system made the idea of 4e's very precise system obsolete.
Does assumed item progression lock in a particular playstyle? No, I don't think it does. Because "availability of magic items" is not a component of playstyle. That is part of the fluff of a campaign setting.
umm....wow. This took me a moment to comprehend. What you're saying here is true, but only from a deeply gamist perspective. Even in 4e having more items would mean having more options...so I'm not even sure it's true there. That's not a bad perspective, just a bit foreign to me.
"Balanced encounters" vs. "whatever the DM thinks would logically show up" vs. "whatever shows up on the random tables" is playstyle. And without robust encounter building rules, the "balanced encounters" playstyle is locked out, at least not without forcing the DM to play game designer and suss out game balance themselves. Which is difficult, and therefor what I pay game designers to do for me.
Part of the point of Bounded Accuracy is to make the game less confined and more forgiving. If "whatever the DM thinks would logically show up" and "whatever shows up on a table" are more balanced, then its not an issue. Ideally, the amount of effort needed to create a balanced encounter drops so low that its not an issue. As I mentioned, this was not a problem in earlier editions (at least, I never ran into people decrying their inability to decide how many orcs was okay.) It was the constant escalation of attack, save, and skill bonuses that started in 3e which brought this on by narrowing the band of foes that were "appropriate".
The second thing I'd like to ask is "balanced for who?" I have to DM 4e differently when I'm DMing my kids than when I'm DMing other more experienced adults. There seems to be a presumption there that "balanced" somehow has some universally applicable meaning that I no longer believe exists. For a lot of people, what you call "balanced" they experience as "samey". Just to trot it out again:
"With fourth edition, there was a huge focus on mechanics. The story was still there, but a lot of our customers were having trouble getting to it. In some ways, it was like we told people, ‘The right way to play guitar is to play thrash metal,’ But there’s other ways to play guitar.” -
Mike Mearls.
On the other hand, assumed progressions are quite easy to adapt to different campaign fluff preferences. Just apply the assumed bonuses as inherent bonuses. As suggested in 4E. And there you have it: both low and high magic item availability campaign settings are supported equally well.
That works for an Un-bounded accuracy (to coin a phrase) system like 3&4es, but not so much when magic items provide things in different directions. Its the stacking of all those bonuses that generates this need for encounter balance in the first place. Eliminating (or restricting) those as much as possible will go a long way to eliminating this balance problem in the first place. (So that they can get on that Fighter-Wizard thing.
)
This isn't an impossible goal, either. There are many Indie games, some weirder than others, where "balance" of this type isn't really even an issue, because it's built right into the mechanics. The ones closest to D&D seem to feature very tightly bounded accuracy.
Meanwhile, that 4E lawnmower you've got running like a dream is great for the main yard, but not so hot for trimming the edges or handling low-hanging tree limbs. 
