I won't tell you not to believe your eyes if you think photos look similar, and as always with photography your mileage may vary. I opted for the 12 Pro myself, but the difference between the 6s and the 12-regular is not what I'd characterize "not a substantial upgrade".
I didn't have a 6s, but can say with a great deal of confidence that the difference between a 7 and the current generation is quite substantial. My wife was using a 7 at the same time as I was using an X, and even leaving aside the multi-camera benefits I could easily tell the difference between her photos and mine at a glance.
The difference between an X and 11 Pro (again, leaving aside multi-camera benefits) was noticeable but much less dramatic, and the difference between an 11 Pro and 12 Pro is minor--mostly challenging-light edge cases.
If you had an 11, XS, or maybe even an X, I might agree that the 12-regular wouldn't be a big step up, but coming from a 6s the quality improvement of any of the last two or three generations of iPhone should be immediately noticeable and I would argue pretty substantial.
Basically, over the years iPhone cameras have been on a gradual asymptote toward the hypothetical ideal camera, and while the last few models are definitely up in the flatter part of the curve, the 6s was still somewhere on the knee.