Responding out of order for better flow.
That sort of removes the reason for dice, doesn't it? Dice offer the possibility of failure in the face of good odds. If the characters are always going to succeed and the dice are just offering setbacks, why not just play a game that eliminates random chance altogether?
Because the random setbacks help generate a different experience each time you play and this helps to keep the game interesting.
I just said the exact opposite of that. These encounters allow you to use monsters at any level, regardless of the magic items the players are using. 4E only allows you to use those that are the perfect CR, because they are so powerful that the magic items are required.
I think you are conflating bounded accuracy with encounter guidelines that do not take magic items into account. Bounded accuracy is what gives a 1st-level PC a non-trivial chance of hitting the Tarrasque, and conversely, a CR 1/2 orc a non-trivial chance of hitting a 20th-level PC. The encounter guidelines are what tell you that the 1st-level PC is likely to get squished by the Tarrasque, and that you are going to need something like 60 orcs to present a moderate challenge to a party of four 20th-level PCs.
The fact that the encounter guidelines do not take magic items into account means that 5e tells you that those 60 orcs are a moderate challenge for a party of four 20th-level PCs regardless of whether they have no magic items, or whether they have 9 or 10 uncommon permanent items, 5 or 6 rare permanent items, 5 very rare permanent items and 4 legendary permanent items between them.
As I said before, if we're going to go by the houserules, there's not really any point in continuing this discussion.
Here is what it all boils down to: in previous editions, such as 3rd and 4th, magic items were expected. The was a certain amount of magic that the players had to have to progress through the game normally. 4th edition was the culmination of this, as some monsters had such large stats tht to defeat them without magic was impossible, regardless of the level of the PCs and their skill. 5E does away with this by eliminating the assumption that characters would gain magic items at all. This makes magic items mean something, as they are not just a gate to higher monsters, but an advantage overall, because the monsters were not created with the expectation that you would have magic items at all. That's the bottom line.
It seems to me that this is a rather odd position for an advocate of 5e to take because the implications of following the rules as written seem to me to be much worse for 5e than for 4e. As mentioned, since the encounter guidelines do not take magic items into account, every magic item that you give to the PCs will make all subsequent fights easier. And if you keep handing out magic items, as mentioned before, over time, the accumulation of magic items is going to make the recommended fights easier and easier, and unless you have players that aren't bored by easy fights, they are going to lose interest.
On the other hand, it seems to me that the 4e system as written keeps things fresh and interesting for the players because the PCs get more powerful, the find new and better magic items, and the difficulty and challenge of the fights keep pace with what the PCs are capable of.
"You can house rule it" Isnt a very good avenue for discussion. Of course you can house rule it. You can do that to anything. This is a discussion of the rules as they are.
And for those of us who think that flexibility, adapatbility and ease of houseruling are key advantages in an RPG, I would just like to re-iterate the point that it is trivially easy to adapt the 4e encounter guidelines to re-create the 5e approach of ignoring magic items when determing what is an appropriate challenge for the PCs. The key failing of the 5e encounter guidelines, to me, is that they are not able to advise the DM on how to maintain the challenge level of his game (if he wants to) after he has started handing out magic items.