You sound as if you feel betrayed by Sennheiser, and I think you really shouldn't.
They just went for a headphone with a spacious sound. The harman target is not an objective but an inter-subjective standard, and not everyone prefers it.
I agree that frequency response is by far the most important factor of a headphone and that it is also very important for soundstage.
But I think it is also a wide consensus that the construction of the phone is at least not irrelevant (driver size, driver angling, size of resonating body).
Look at this review for example:
https://www.audiosciencereview.com/...mark-levinson-no-5909-headphone-review.35292/
Amirm said there that even though the phone hits the harman target really well, he is still not satisfied with the soundstage. I can also tell you for sure that my Etymotic ER4XR (an IEM) does definetely not have the same soundstage as my over-ear phones.
As they went for a construction which clearly is supposed to support a spacious sound, I would rather view it as consequently playing the strength of the phone.
Personally, I would rather pay for construction than for tuning, as there is always the option to EQ.
If you are convinced frequency response is 100%, even for soundstage, you dont even need to buy a 560s. There are IEMs for under 50 Euro which hit the harman target almost perfectly.