I agree with you. This is a very important feature especially for old or budget devices.
I tried to solve this problem using my own manager and this method of texture loading. But it’s a lot of work and bunch of issues in this realisation.
7 years ago I worked in one company where we made a CI system that builds resources with different resolution and engine that cas use all this textures. But it was Adobe AIR and we had more access to resource loading into the engine.
This issue already solved in Starling framework (for adobe flash/air) : http://wiki.starling-framework.org/manual/multi-resolution_development (screen coordinates in the abstract points and developer can choose wcith content scale factor he want use in the game and for a texture loading)
In theory this feature can be implemented using mipmaps where developer can chose of mipmap level for texture in the fragment shader depend of screen size. But mipmap can’t be x1.5 or x0.75.
I hope that this feature will be implemented in Defold.