I don't think their games are necessarily becoming more simplified, it's more that they seem to focus on areas of the games which would help make them more "mainstream" (for example, Fallout 4 made crafting and upgrading more complex compared to 3 and NV, but this is similar to other AAA games), while focusing less (and thus simplifying) other areas.

This is not necessarily a bad thing, I personally think Fallout 4 improved over 3, and Skyrim over Oblivion (though NV and Morrowind are still better). But this also leads to disasters like Fallout 76.