I don't have an investment in RF-S body/lenses so a theoretical question from me...The addition of longer Sigma RF-S lenses is more a wish than a prediction. Certainly, a Sigma 70-200 f/2.8 DG DN S could serve as the long part of the RF-S trinity, but compared with the 10-18 f/2.8 DC DN C and 18-50 f/2.8 DC DN C, it's large and heavy. Hence, the desire for a small and light 50-135 f/2.8 DC DN C lens.
If short 2/3 of the FF Sigma trinity is the 16-28 f/2.8 DG DN C and 28-70 DG DN C, then the 70-200 f/2.8 DG DN S is still comparatively large and bulky and might better be replaced with a smaller and lighter 70-200 f/4 DG DN C. (I'm 74 years old and trying to lighten my load.)
If this sounds weird and obsessive, consider that Canon does offer two complete FF L trinities, one where all the members are F/2.8 and another where all the members are f/4. (I own the EF trinity in F/4 and can adapt it for use with my R6-2.) So does Olympus/OMD. Canon doesn't offer a trinity for RF-S but apparently may be introducing one.
Did that answer the question you asked or did I go off on a tangent???
I can see that a RF-S equivalent to 70-200/2.8 would be useful if light/cheap/small. If that isn't possible then the existing RF lenses would work well even if heavier, more expensive etc.
My own mix of EF/RF lenses covers 8-500mm with three f2.8 and one f1.4. It suits my shooting rather than a specific "trinity" of sorts.
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