The "magic items are optional" "promise" is not, and never was, a promise that magic items wouldn't make a character more effective.
It was (and still is, as far as I can tell) an intent to craft the game in such a way that if a DM chooses not to include any magic items at all it won't break the game (and, conversely, that a DM loading the party down with magic items will also not break the game).
This is something that was impossible in both 3.X and 4e without incorporating some sort of compensating measures.
Yes, it's called "eyeballing encounter difficulty" - something that most DMs that cut their teeth on 2e and earlier editions learned to do.
3e's CR system was simply a way of making the DM's job easier by providing better guidelines given certain assumptions. Throw the assumptions out the window, and you're no worse off than before.
4e greatly simplified the underlying math, so much so that adjusting monster selection for PCs with less powerful (or no) magic items is almost trivially easy. PCs of level X with no magic items can take on monsters of level 0.8 * X at standard difficulty. If they do possess magic wapons, armor and neck slot items, add the "pluses" together, divide by three and divide again by the number of PCs, and increase the level of the monsters they can take on by that amount.
Fundamentally, math is math. 5e seems to be getting around the problem by making the mathematical bonus from most magic items fairly small - capping at +3 or so. However, unless they provide some kind of guidelines for adjusting encounter difficulty for powerful magic items, a PC fighter armed with a +3 weapon (such as a
vorpal sword), a
belt of storm giant strength and a
pale green prism ioun stone is going to have a melee attack bonus +7 higher than if he did not have these items. Of course, with bounded accuracy, maybe it simply means that past a certain point, additional attack bonuses don't matter any more because you're already only missing on a natural 1.
(As a side point, AC bonuses are actually more valuable, since attack bonuses have diminishing returns as they get relatively higher, and eventually have no significant marginal benefit once you get to the "only miss on a natural one" stage. However, AC bonuses have increasing returns as they get relatively higher, until you get to the "only hit on a natural 20" stage.)