Those tools are already available for years, if you do not want to distribute your models for free then simply don't use 3D viewers.
The people using Sketchfab probably don't mind their models get distributed for free, they sell license and as long as they feel enough people buy those license then they don't mind all the free downloading. At least that's what I assume, otherwise I don't get the logic using 3D viewers?
Note, in order for a computer to display 3D graphics there usually needs to get data (vertexes, UV cords, textures and shader data, etc.) tot the V-ram on your GPU from where it can be rendered. Sites like Sketsfab (using 3D viewers) distribute this data to the users computer via the web-browser where an embedded app delivers it to the GPU for rendering.
So the simple act of viewing such models puts it already on your PC. The only thing holding you back from opening it in Blender or whatever 3D app is the format the data is in. However, there are plenty of smart people who know about the techniques behind the encoding and decoding of this data and they provide decoding apps so you can convert this data to regular OBJ or FBX format.
Anyone is free to transmit/distribute his work over public network for free but I find it a bit strange as its kind of opposite of what most license describe?