I think one of the reasons Apple continues to sell the Mini, despite the fact it's clearly not very popular, is that it allows them to upsell. They can honestly tell people that it is possible to get a new Mac for $500. Then, once in the store, it's possible to upsell on an SSD and RAM upgrade, or show them the gorgeous and more powerful iMac, or the affordable and portable Macbook Air, etc. Without the Mac Mini, the least expensive Mac is the 11" MBA at $900. There is a lot of distance between $500 and $900, especially psychological wallet-parting distance.
For this reason, I think Apple's main concern with choosing Mac Mini components is price. They will put the least expensive CPU into the base model that will run OS X reasonably well, to keep the price at $500 or less. Right now, it seems like Core M actually costs a premium over other more powerful chips with a slightly larger power consumption. Thus, it probably won't be in the Mac Mini.
Since Apple gets a better deal with volume, whatever CPU they order the most of will likely go into the next Mac Mini, since it will probably be the cheapest. This makes sense, since the 2014 base model has the i5-4260U, which is the same CPU has the very well selling 2014 MBA base model. I'd wager Apple ordered more of that CPU in 2014 than any other Intel CPU. Prior to that, the Mac Mini typically inherited components from the 13" MBP, when that was the top-selling Mac. Since I think the MBA will continue to be a very good seller, unless the price of the rMB drops significantly, the next base model Mac Mini will have the same CPU as the MBA.
For this reason, I think Apple's main concern with choosing Mac Mini components is price. They will put the least expensive CPU into the base model that will run OS X reasonably well, to keep the price at $500 or less. Right now, it seems like Core M actually costs a premium over other more powerful chips with a slightly larger power consumption. Thus, it probably won't be in the Mac Mini.
Since Apple gets a better deal with volume, whatever CPU they order the most of will likely go into the next Mac Mini, since it will probably be the cheapest. This makes sense, since the 2014 base model has the i5-4260U, which is the same CPU has the very well selling 2014 MBA base model. I'd wager Apple ordered more of that CPU in 2014 than any other Intel CPU. Prior to that, the Mac Mini typically inherited components from the 13" MBP, when that was the top-selling Mac. Since I think the MBA will continue to be a very good seller, unless the price of the rMB drops significantly, the next base model Mac Mini will have the same CPU as the MBA.