As far as I could see not much were written in the forum about this and the assignment/copy constructor seems to turn the texture to an image before creating a new texture. I am just wondering if there is a reason why you are not using directly the copy functions for textures provided by OpenGL? As far as I can see they exist in very old versions of OpenGL as well so it should be compatible.
While we are at it can't you create a update function like you have for images but for textures instead? Where you can copy a rect from one texture to another without going trough render textures? Would make things more simple.
Do you need a usecase? I want to generate some textures at runtime and currently I have to go trough sprites and render textures to get that. Works so far but later I can't move that data to a texture efficiently without having to make it into an image first which I will need.