If you have an XP/monster designation that communicates with an XP/encounter difficulty system, then you need a Magic Item/Power system.
I am not convinced at all. You can totally disjoint the XP system and the Magic Item system, so that the amount of magic items can be totally decided by the gaming group.
It's not even that difficult: first you just design your XP/encounter system assuming ZERO magic items are ever used; second you design a measurement system for magic items (gp are a good unit of measurement because they naturally tie-in with treasure for those gaming groups who want to be able to buy/sell magic items, the other groups who don't like magic items market like me, can still use them as a measurement unit without implying they can be found on sale) and when a gaming group uses a certain amount of magic items then the DM gives the monsters additional abilities or bonuses of equivalent value.
Throwing up your hands and saying "DMs can handle it" just leads to frustrated DMs (especially new DMs).
Well, new DMs should really start running the game at level 1. Which means no magic items.
Then I guess that
some DMs will be frustrated by being on their own, but
most DMs are frustrated by being told how to do it when the system itself isn't good enough.
I'm not greatly concerned: nailing down a Wealth/Level system is easy math, and is crucial for training new DMs.
"No magic items" is a beautifully perfect nailed down system with the easiest math ever
Any assumption is going to attract more criticism than favor, at least on the long term. Maybe on the short term assuming a certain level of magic items (presumably a compromise) can meet the favor of the majority and alienate only those who like a grim'n'gritty game or a monty haul game, i.e. the extremes. But on the long term even a compromised middle ground is going to frustrate people, because it's still an assumption on how they should play the game, and if the system isn't flexible enough, it locks all their campaigns to the same level of magic items... this is exactly what happened in 3e, at the beginning there wasn't much hate of 3e magic items. It all started when people began to notice that they were stuck with always the same level, or the game started to be hard to play and run.