Tilers (mostly mobile and Apple) generally expose the ability to read & write the framebuffer value pretty easily - see things like GL_EXT_shader_framebuffer_fe

Tilers (mostly mobile and Apple) generally expose the ability to read & write the framebuffer value pretty easily - see things like GL_EXT_shader_framebuffer_fetch or vulkan's subpasses. For immediate mode renderers (IE desktop cards), VK_EXT_fragment_shader_interlock seems available to correct those "concurrency" issues. DX12 ROVs seem to expose similar abilities. Though performance may be hit more than tiling architectures. So you can certainly read-modify-write framebuffer values in pixel shaders using current hardware, which is what is needed for a fully shader-driven blending step.