What possible definition of “game breaking” can you be using, here?
What possible definition of game-breaking? Game-breaking in terms of trivialising encounters, of course. A full caster with the very rare DC increasing spell book/censer/what have you can have a spell save DC of 20 as early as Level 9, and at that point they can completely derail what was supposed to be an epic and challenging fight with a single save-or-suck spell. The beholder, a CR 13 creature (meaning it needs to be a challenging encounter for a party of four level 13 PCs) has a Strength of 10 and a Dexterity of 14, meaning you could make it almost guaranteed that the Beholder will fail a save by using a Strength save or a Dex save spell. Suddenly, your epic confrontation at the end of the dungeon becomes a "I restrain the Beholder with a
Maximilian's Earthen Grasp and the Fighter proceeds to pummel the poor thing." And unlike Warlocks, who can only do this two times per encounter with a very limited spell list, the full caster will have a far larger selection of spells and spell slots, meaning that each monster will get at least one laser-precise save-or-suck spell their way.
Sure, you can work around this. Legendary resistances, multiple monsters, monsters with abilities that specifically counter the wizard... But all this designs
around one specific feature of the full caster with the high spell save DC, and a feature that requires everything else in the game to take it into account or be trivialised is the definition of game-breaking (an analogous case are format-warping cards in trading card games, where cards so powerful that you either have them or add an answer to them in your deck are banned because they make for an unfun gaming environment. I know the case of competitive TCGs is different from collaborative RPGs in tables without a "meta", but in my experience neither the DM nor the players have as much fun as they could when encounters get trivialised).
The funny thing is, WotC had hours of footage that proved the game-breaking potential of giving DC-increasing items to full casters in the form of Scanlan's Hand Cone of Clarity in Critical Role Campaign 1. This was a homebrew item that was a holdover from the party's Pathfinder game that increased Scanlan the Bard's spell save DC by 1 (so not even close to the +3 provided by the Very Rare version of the items in Tasha's), and Mercer visibly
hated the item and how it allowed Scanlan to guarantee that some of his NPCs will fail their saves. Later in the campaign, Mercer would make sure that the item's use was minimal by requesting that Scanlan declare that he's casting via the hand cone (which Scanlan's player, Sam, often forgot to do), and despite that the item was clearly overpowered and completely threw the balance off of some of the encounters.
Or the designers, lead by the same guy that wrote the PHB, have realized that letting casters raise thier DC won’t break the game.
Generally, assuming abject idiocy from professionals based solely on them doing a thing you wouldn’t have done doesn’t lead you to accurate conclusions.
Well, of course we cannot know who exactly designed which portion of the book, and it's true that Crawford is the lead designer in both books. Nevertheless, I don't think it's unfair to assume that the designer team and their design philosophy changed radically between the two books, especially since none of the other designers and writers are the same between the two books (and more importantly, one of the co-lead designers, Mearls, is absent entirely in Tasha's). This isn't just the case with spell save DC increasing items, but with class features having use limits based on proficiency bonus and not primary ability modifier (effectively limiting their use based on campaign tier and not character advancement in a specific class, and also giving a hidden boost to multiclassing) and the obvious sacred cow that was slain in racial ability score bonuses. Signs of this change only strengthened with new changes such as monsters' spells becoming custom abilities that cannot be counterspelled (greatly reducing the spell's value), and WotC even retroactively cutting out several pages' worth of content from new prints of Volo's.
Also, just to be extra clear, I don't think the design philosophy they chose is
in itself an abjectly idiotic/unbalanced/badwrongfun direction. I think the problem primarily came from pivoting the game's core design philosophy halfway through the edition, and that meant that old content was just designed with different priorities in mind and having the two content from different eras felt
awkward if not janky. The new direction gave them advantages in some ways (no longer forcing players to have a subpar stats just because they chose an unorthodox race/class combo, for example), but it also hampered them (and I argue will continue to hamper them) in new ways. Saying they will no longer give any cultural racial traits means suddenly races that became iconic due to their cultural features get very odd racial traits (the gun-toting Giff of spelljammer getting all melee-focused traits in the penultimate UA, for instance), and means that WotC has effectively closed off what could be very interesting design avenues. Both mechanical balance-wise and story-wise, the game's two eras feel too different to exist simultaneously. A game designed from the ground up with the new principles would probably feel a lot less awkward and I would probably enjoy it immensely. But in my current home game, for instance, I felt obligated to nerf Twilight Cleric so that it wouldn't outshine anyone who played pre-Tasha's options, and I outright ban most of the spells and magic items post-Tasha's (such as Silvery Barbs and these DC-increasing items for full casters). While I could implement almost anything created by WotC in my game pre-Tasha's, now I instinctively feel like I have to vet the new things they release. So no, I don't assume abject idiocy from the designers, it's just clear that their design priorities shifted. And when I try to include content with different design philosophies in my game, they don't work well and that leads to my feeling of imbalance.
