The designers said it. And they wrote that in XGTE.
They did not. It was just quoted from XGTE that they said outright magic items are expected to be in the game. "The D&D game is built on the assumption that magic items appear sporadically and that they are always a boon." That's exactly what I've said all along - the designers expect they will be there, but because their distribution and quantity is random based on tables, they didn't design the math around them because they'd have no way to judge what the math needs to accommodate. But they absolutely acknowledge magic items will 1) be in your game, and 2) their appearance will be sporadic, and 3) they will be a benefit to the party.
It then ALSO tells those players, "Magic items can go from nice to necessary in the rare group that has no spellcasters, no monk, and no PCs capable of casting magic weapon. Having no magic makes it extremely difficult for a party to overcome monsters that have resistances or immunity to nonmagical damage. In such a game, you'll want to be generous with magic weapons or else avoid using such monsters." So your claim WOTC never tells people the game changes meaningfully if they have no spellcasters AND no magic items is false - they do tell them.
AND the new version of the game appears to have removed resistance or immunity to non-magical weapons. So any issue which could have arisen from that part of the issue was already resolved.
New players and DMs have no reason to go into the game thinking a total lack of both spellcasters and magic items is the norm or expected to function well without adjustment. At no point does the game suggest to players or DMs this is how the game functions. Nor is there any meaningful number of players complaining, over a decade period of time, that they do this and the game crashes for them. It's just not a common problem. It's not something which must be solved. It was a hypothetical issue which never arose. People who want to do that either play another game, like the 5e version of Lord of the Rings which intentionally adjusts 5e to accommodate those assumptions, or another game, or they adjust 5e to work for them - either through differing player tactics, or a DM adjusting challenges. And there's no great number of people out there saying this is some issue which is in need of fixing.
Now that we've definitively established that, and I said I was asking for the last time for the evidence you never produced, I guess we're done here?