Why is there no glGetTexSubImage? There’s glGetTexImage, and glTexSubImage, but no glGetTexSubImage. Seems like it would be useful to have for modifying portions of a large texture. You could read back a small rectangle of texels, do some modification of the data on the CPU, then write back the portion with glTexSubImage.
You do have a point but I dont think many people needs it.
I guess most instances where you would want to modify a part of a texture and need to blend the new part in with the old texture can now be handled with the render to texture extension.