that's just part of RTX remix and somewhat the point of that tool. After all you need to get proper RTX materials in there and why not upres the assets as well while you are at it?
I'm not sure how easy that is with RTX Remix. IIRC the assets aren't actually added to the game itself, but rather they get injected into the render-pipeline.
You have to use new assets for these old games if you really want to make the most out of ray tracing because PBR materials are necessary to better simulate the way light and reflections bounce off a surface.
I disagree that you really need to put much effort into updating the materials to get benefits from rtx. HL1 RTX looks pretty great with only the bare minimum PBR remasters, because PBR and RTX are 2 separate things that both improve a game's look. PBR materials help rtx just as much as PBR materials help regular rendering, and rtx helps PBR materials only a little bit more than rtx helps regular materials. All of the effects used in PBR materials apply the same to regular rendering as with RTX.