2.1 is the "lowest common denominator" these days. Even mobile* and Open Core GPUs can be expected to support it. And, as far as performance and visual effects go, for a 2D game, you should be able to do anything in 2.1 you want. That said, the most important thing to me is that my friend on 3.3 can ...

My laptop only supports OpenGL 2.1. I'd at least still be able to play on my desktop if Factorio went with 4.1, but a friend I often play multiplayer with is stuck on a machine that only goes up to 3.3. I know the benefits that later OpenGL versions offer, but I still hope that 2.1 will continue to ...