[wim]

I guess a 28-70 covers 3 important focal lengths: 28-35-50. Not sure whether the 50-70 range is that useful.

Just as useful as the 35 to 50 part, as the step is 1.4x linear, or 2x area-wise.

A 28-85 would make more sense IMHO.

Only because there is more range, and the step from 28 to 50 and 50 to 85 is both 1.7x linear, yes.

Traditionally, manufacturers have been producing these two main zooms: 24-70 and 70-200
Such a combo still lacks any UWA focal length, so you'd need to add a 16-35 or equivalent, thus a 3rd lens.

Traditionally they have been producing UWA zooms as well, once it became possible.  That is what the 16-35 has become, just that initially the lower end often was a bit less wide, 17, 18 or even 20.

Don't forger that for a long time a 20 mm was the last step down UWA wise, unless you wanted to go fisheye.

I argue that the following combo would be much more useful: 16-50 and 50-150
With these 2 lenses you get a much more useful range covered without the need for a 3rd lens.

16-50 is 24-75 APS-C, so what is the difference?  Same with a 50-150, which becomes a 75-225.
These lenses do exist, just not for FF. And a 16-50 would be rather difficult to make.

The 150-200 range doesn't matter much IMO.

That depends on one's personal preferences and use. It is indeed an opinion. I would prefer a 100-250 or 100-300 over a 70-200 or 50-150.

Why is it that no manufacturer released such lenses? Would a 16-50 be much more difficult to design than a 24-70?

Yes.

Even a 35-70 was difficult to do in its days, but that was really a proper standard zoom lens, with 1.4x linear steps going from 35 to 50 to 70, with area-coverage of 2 : 1: 0.5. There was method in this madness.
28 mm was added later, when it was possbile with new glass and computers, because that was considered a true WA.
When things improved further, the range was extended further, to 24 mm, which back when was already considered UWA, with an AoV of 84 degrees diagonally.

It really gets very complex when designing rectilinear lenses which have an AoV of more than 90 degrees, IOW, on FF any lenses with a shorter FL than 22 mm.

IOW, the current 16-35 mm class zoom lenses are really quite incredible considering their imaging possibilities.
Combining this with a "normal" AoV, or even tele AoV, makes it even more complex, because image corrections for the short and tele end of such a zoom are for practical reasons contradictory, plus you'd need to design an image forming part of the lens that is extremely sharp to have it magnified over such a long range and still give good resolution and correction of aberrations.

Therefore, it is easier to design a lens in the 3 classes that now exist, namely a WA-zoom, a normal or standard zoom, and a short to medium tele-zoom. It just makes for the least number of compromises for the best results, and also allows for maximum apertures to be relatively large compared to wider range zooms.

It gets a bit easier with the shorter mount distances with mirrorless cameras, but that only helps so much, as other restrictions come into play. This is also why software for correcting lens flaws is playing a continuously larger part as well. Unfortunately, up to now, it still means that resolution is not optimal with such corrections, although I expect things to improve drastically over the next decade and beyond.

Kind regards, WIm