The mx250 is not a "middle class DGPU" because there is no class below it. It's Nvidia's weakest dedicated graphics card they continue to sell, and it's a pretty terrible point of comparison. A better example would be the GTX 1060, a middle-class dedicated GPU that made it's way into many budget gaming laptops 7 years ago. It is faster than the M1's GPU.

The Mac has a very dedicated audience of video and design professionals who are going to be left empty-handed here, even if they double or triple the amount of GPU cores in the SOC.

Gaming laptops I assume.

M1 GPU beats most other integrated GPUs, but that doesn't magically make the demand for more performance go away, and that demand is likely to increase as there become more and more non-gaming applications for GPUs (machine learning, video editing, etc).