24-70/2.8 and 70-200/2.8 are part of so called “Holly trinity” set-up. (14-24/2.8 will complete it). While this reference is usually related to original Nikkor lenses, it also represents needs of commercial photographer, one who is shooting events, weddings, corporate portraits, familly, kids, pets and similar assignments.
Those photographers needs fast AF, solid image quality from widest aperture, good build quality, durability, some sort of weather sealing, in other words – reliability in every situation.
While prime lenses will mean another small step in image quality improvement and more importantly faster aperture (usually), versatility of the zoom is more important for them, because their work is very much about speed and readyness. You’ll see them many times to literally throw their camera anywhere nearby, lift another body and keep shooing. Bride will never repeat her walk, ring will never be put again on that finger, father will cry only few seconds, etc.
Holly trinity line-up is made for that type of photographers at the first place.
Superzooms are consumer graded lenses. They are designed for those who doesn’t like to switch lenses in the field, who are shooting mostly in full daylight, who doesn’t need superfast auto focus and who can live with slight image quality compromise.
If such a lens will be at least. f/2.8, I believe that many above mentioned pro photographers will use it too, but unfortunatelly superzoom lens is rather slow.
Superzoom is great for traveling, but the biggest question is, why would you need it, considering bridge camera offers, that are cheaper, smaller and often full of extra features, such as 4K video (Sony RX10ii e.g.)
Superzoom is not a good option for interior scenes (need lot of light), architecture (usually pronounced distortion at wide end), action/sport (to much glass to move during focusing and thus slower auto focus) and images where imagequality is priority.
In general, you are comparing “pro” with “consumer” grade products.
I have original Nikkor and Canon holly trinity set-ups, but I have heard about Sigma and Tamron being also very good optically, at least their recent 24-70/2.8 and 70-200/2.8 offers (not sure about AF consistancy).
Problem with AF and third party lenses is usually not in the accuracy, but in consistancy of AF accuracy. In other words, you can have 4 shots in perfect focus and 1 shot with shifted focus, but problem is that you never know when. that 1 shot will come. (That’s at leats my experience with Sigma and Tamron lenses that I had chance to try).