The use of SFML has nothing to do with the fact that a current version of Windows XP is required to install the combat demo. We use SFML precisely for cross-platform portability. SDL doesn’t add anything relevant to our needs that we haven’t already covered with SFML.
Advanced visual effects is a relative term; Battle Brothers makes use of shaders for some visual effects and to add variety to a limited selection of sprites, e.g. by changing their saturation and color, which allows us to get away with drawing fewer variants manually. Since we don’t have the time and resources to add compatibility for all possible hardware configurations, we concentrate on those that most people use (see: http://store.steampowered.com/hwsurvey/ – also keep in mind the game will be released in 2015). The game requires an OpenGL 3.0 compatible GPU because it made things more efficient for us and allowed us to concentrate more on the game itself rather than trying to get it to run using different rendering paths for different GPUs on OpenGL 2.1. It was a conscious decision to potentially forgo compatibility with video cards released before 2008 this way.
I’m sorry the game won’t run on your laptop, but as harsh as it sounds, we have to set priorities with our limited resources. Having the game run on old laptops with an OS that was released in 2001 and isn’t updated to a current version can not be one of them.