The prior Blinn-Phong materials could be approximated for non-PBR viewers using the current PBR layers and baked server or client side. The current setup using both at once is sorta asking us to have 6 textures per unique face, and seems silly.
Without 100% understanding the color theory behind them: the Diffuse layer seems to be a combination of metallic+base, emissive is the alpha for emissive blending at least, normal is the same, and the metallic/roughness are related to glossiness/tint.
For builds which have non-default Blinn-Phong textures set, just do whats currently being done.