I would say that 5e represents "sufficiently good quality" design. The quality of the core engine and its architecture is strong for its design purpose and primary modes of gameplay, and this is what does a lot of the heavy lifting for the system. The problems or weaknesses tend to come from the variable design quality of the accessories or even smaller nuts and bolts that are attached to that core engine.
First, what, exactly, is "the core engine and its architecture"? Because you could say that about 3e if that phrase means "the d20+mod method." I would not call 3e any kind of good design on nearly any level. The designers of 4e, 5e, and PF2e have all said things to the effect of "yeah, 3e was pretty bad." On the flip side, one of the most common and (IMO) justified complaints about 3e was that its CR system sucked, and IMO/IME 5e has mostly followed in 3e's footsteps when it comes to CR. So the question of demarcation needs an answer before I can respond to the statements made.
Second, what purpose is it fulfilling? I cannot respond to a claim that X does what it is for if I don't know what thing it's supposed to be for. That's why, in my linked post, I referenced things like the three pillars of play. Those seem pretty clearly like a stated purpose, "these things are essential to the D&D experience," and yet the design is patchy at best for two of the three (exploration and socialization), doubly so when you factor in that every class has specific tools for the third (combat) but many classes offer either little to nothing unique for addressing the other two pillars, or what they do offer has a serious trivializing effect on them.
Finally, if you consider the design to be strong, where is it strong? What things does it do that provide great support for something? As others have argued, strong design is not "make it sufficiently minimal so the GM has to design their own solution," that's just not designing something in the first place. I gave my several, specific, clear examples of issues I and others have dealt with in 5e, so that people could see I wasn't talking in airy-fairy generalities, nor solely in pure anecdote.* If you assert the opposite claim, it would be very helpful if you had examples of your own. (I won't expect anyone to match my long-windedness and excessive circumlocution, but having some concrete stuff to discuss really would help.)
*Though I do just absolutely
love the Morton's fork that people are so rip roaring eager to deploy. If you base your analysis on reasonable assumptions and objective facts, it's white-room theorizing and thus utterly wrong and irrelevant. But if you base it on actual lived experience, testimony from other players, and statements made by other people online, because the perspectives of a few finite players are subjective and thus irrelevant. So no analysis, other than praise, is valid! It's a
brilliant "heads I win, tails you lose" approach. Very effective for dismissing any and all issues, especially when paired with softer or subtler versions of the Oberoni fallacy.