I wonder if there'll come a day when we'll be able to drag and drop old games into a real time path-tracer or similar global illumination renderer. I'd imagine processing the scene brushes and geometry in a BVH or similar structure is pretty do-able algorithmically. The textures would need to be migrated to a proper physical material system, and they'd look awful anyways so they may as well get redone from scratch. Any billboarding, sprites, and flat shader effects would have to get scrapped and redone by hand also. After all that the light propagation may end up looking perfect, but I suspect the environments would look even more artificial without the graphical vaseline of low res aliasing, bilinear filtering, mip boundaries, fuzzy radiosity, blob shadows, etc.
It would seem to me to be a better use of time to just recreate from scratch the portions of the game that are cool than trying to port everything over and have it look shitty. I'd rather have a fully modernized HL1 tram ride than have the whole game ported to Source2.