Well this idea is extremely theoretical as people nowadays rarely accept a DirectX7 level of graphics quality. So it may be true that the absolute minimum requirements 3DGS has are lower than for e.g. UE4. That's not some Acknex optimization magic though but simply the absence of many features. Just try to apply normal maps to all of your surfaces (which modern internal GPUs can handle just fine) and start to welcome practical and performance problems in 3DGS. I mean it in theory is not impossible to create a game that consists just of some textured polygons with a little bit of lighting. But practically speaking it's going to be very difficult to actually convince people of such a level of quality...

I mean even two-man-shows like those simulator games (that sell very well) use all kinds of advanced shader work and other techniques (as e.g. managing seamlessly driving your bus through an entire city otherwise would be a performance nightmare). The same goes for those horror games that pop up since Slenderman entered the fray. Going lower also means looking worse than most 3D smartphone games. That's why I don't see a market for games on an ultra low technical level. It's not that a modern engine is slow - they enable you to use advanced features at a fantastic performance and with this I don't mean just "luxury" features like tesselation but simple shader or lighting work. Things like that have become a no brainer today and e.g. applying normal maps gives a huge improvement in visual quality while costing very little performance - even on modern iGPUs. A halfway modern version of those things even can run BF4 flawlessly with 30-40 fps. That's why I don't think there'll be many that will take a downgrade to BF1942 graphics without a grain of salt...

So if you're really aiming at the 8GB IntelHD customers still go for a modern engine rather than 3DGS. Or maybe switch to the mobile smartphone market entirely as you have many chances to target those people as well as others here. With a modern engine you even could target both the smartphone and PC crowd via multiple publishing options - this sounds like the best option to me...