It creates additional work for the DM having to balance for rules skewed far in favor of the players. What would be the point of switching from 3E where this was exceedingly prevalent to yet another game system that creates major balance problems? I hope I doesn't get included myself. I hope they keep things very toned down. Monster Manual was not built to handle PC groups with +3 armor.
If the DM decides to hand out lots of +3 items, then that's the DM's choice. The DMG should make it clear, and then provide instructions on how to balance it.
The Monster Manual was designed to cater for the default 5e game, in which magic items will be rare and intentionally breaking bounded accuracy. That is, a PC with a +1 sword should be able to notice that they have an important and powerful weapon. Having a magical weapon should be special.
But that's not everyone's game, and the DMG should cater for people who want something more like previous editions. Given their level of execution on the PHB and MM, and the fact they've previously said that guidelines for DMs who use more magic items will be in there, I am currently reasonably confident that they'll include something that works.
That way, everyone can be happy. They won't be, of course - we're talking about gamers here - but the potential is there
