Exophase
Nothing good will ever come of Exophase.
Doesn't that depend on what's being rendered though? Stardew Valley is pretty simple graphically.
Retro-style 2D games probably use much simpler shaders and have a lot less geometry, but they might not actually be that light in terms of pixels. A lot of them are rendered at much higher resolutions than they appear and Mesa (a not very optimized reference renderer) likely doesn't have much or anything in the way of special-cases for 2D primitives. That and 2D could be using alpha blending for sprite translucency, which is faster on 3D hardware than fragment kill but slower in a software renderer.
Just trying to say, this sort of stuff could be a lot faster but might not be. But chances are all around pretty low that anything will really run suitably well with software rendering, especially considering that the game still needs CPU time.