The original Metro 2033 was pulled from sale when the Redux version was released but as usual with this kind of thing it's not a clear cut improvement. Features from the sequel were backported, stealth mechanics were heavily changed, enemy placement and patrol routes changed, scripted sequences were altered, and so on. It's a very different experience.
If the publisher still insists on not selling the original Metro 2033, I hope it could at least be added as a bonus with purchases of Metro 2033 Redux, as GOG has done for other games. People who want to experience original PC version of this game are currently forced to resort to dodgy resellers of old Steam keys, or piracy.