If you can't even store them in vram, you'll have massive performance issues as you fetch them from system memory. Imagine having to go back and forth from System memory to VRAM multiple times to draw a single frame, your framerate will dumpster.

If they are already in VRAM and fully stored, along with their full mipmaps, the performance hit in the pixel copying is much smaller (in fact, it's quite negligeable on modern GPUs).

There seems to be a lot of things you're missing here.