Look at it from Apple's perspective. They can either do what they did in 2011 (include a discrete GPU in the high-end model) or do what they did in 2012 (no discrete GPU in any Mac mini). First, adding a discrete GPU adds $100 to the retail price. That almost certainly would mean fewer sales with a dGPU. Second, providing space on the motherboard for dGPU is a design constraint that requires compromises to the rest of the system (as does any design constraint). Third, it adds some risk and complexity to an already complex supply chain.
If iGPUs were inadequate for a significant number of possible buyers (as they were in 2011), then Apple would bite the bullet and include a dGPU in the high-end model. However, Apple decided that the iGPU of 2012 was good enough for the high-end model. Now it's already dramatically better than that. So there is no way Apple will take what is from their perspective a backward step.
Apple are phasing out dGPUs from all Macs -- except the Mac Pro. The Mac mini and MacBook Air are already dGPU free, the MacBook Pro has only one model (out of six) with a dGPU and that one will go dGPU free with either the Broadwell or Skylake model. Apple started phasing out dGPUs from the iMac line in 2013. This process will only go in one direction, the same direction that all progress in integrated circuits takes: ever increasing integration.