The replay file is just a record of entity locations and object properties, it does not contain the meshes and textures. The executable file itself is not the main problem, with patches there are changes to terrain / ships both in mesh and textures. If the client would not back up every mesh/texture from every patch, you would never reach backwards compatibility ( without more or less severe graphical abnormalities ). I would not want the client to do this, it would increase the space the game occupies to much ( I got precious space on my ssd ).
The meshes are small files, the texture's make up the most space. A possible workaround would be to add an option to keep meshes from previous patch and allow storing of selected textures ( which are either all low quality ~ which is smaller, or highly compressed one's which due to compression would look lower quality ).
Or maybe I am wrong?
No, you are very correct in your assumption, I was not thinking about changes at the time of my post. But, for those that do have enough space I can only imagine it would be like having a separate game installation for each update in order to play the relevant replays so that would rule out this ever to happen.