Yes, it's very sad that there is no quad core option now. However I don't think it was Apple's choice to make :(
If you check the aforementioned I5-4260U, I5-4278U, I5-4308U, I7-4578U CPUs you'll find they all use the BGA-1168 socket and the I7-4578U is the newest and highest spec'd chip for that socket. In order to sell a quad core machine Apple would have to use a different motherboard, which isn't really sensible for what would no doubt be a low volume machine, even as Minis go.
The quad core chips at the moment use socket BGA-1224. (and have a 35W TDP rather than 28W, but that shouldn't be a show stopper).
I'm sympathetic to the argument that you can build a PC cheaper. My current (i7 4790k) and previous (i7 860) OS X work machines have been home build Hackintoshes using the fastest chip available -- and then overclocking it -- at a fraction of Mac Pro (or even iMac) price.
However, the iMac is very small unobtrusive and quiet. These new ones also use *6* Watts when sitting idle (http://support.apple.com/en-us/ht3468), which is what most home servers do most of the time. My home made PCs use around 75W sitting idle. Over the course of a year that's a difference of 600 kWh, or about 100 quid less electricity for the Mini. As the 1.4 GHz one only costs 400 quid in the first place, you're probably ahead vs a home made PC within two years.