This seems like pure speculation. The relative number of people below this spec is probably not worth it for them to focus on this. Besides their explicit support for modding allows them to improved sales value. Consider for example Skyrim. It was re-released so many times and people kept buying it and mods allowed it to look great even years after its release. I think by narrowing their scope they can focus on development of a good core and by leveraging their mod community it can run on older or higher hardware. Win win in my opinion.
Don’t think Bethesda is focused on making their gaming look specifically bad just to make it run on older hardware. Similar to all other companies there is a minimum spec. I do think that having such great mod support allows for this to happen which is great.