So basically, apple hasn't improved their MBPR in 3 years.. ok...
Look at it the other way around, Apple made a really good laptop that doesn't need to be replaced in less than 3 years, you made an investment that worked out really well. I'm perfectly happy with '12 rMBP and I won't be upgrading until Skylake or Cannonlake come out. I'll explain why later.
Since 2012 and 2015, there's not much to improve in that span, Intel can't add more than 5% in general CPU performance every single generation. Memory hasn't changed that in several years for laptops and so on.
Apple did improve in areas that have changed such as PCIe flash storage which is now nearly 2GBps read, 1.5GBps write, wireless has been updated to 2x2 AC, you got Thunderbolt 2 ports and so on. It sounds like you don't care about these and therefore, it won't be a big upgrade for you.
Battery technology hasn't changed much in several years either, so you're not getting a big jump.
Frankly, I would honestly save the money and wait until you get the laptop that you really want. Here's why I'm waiting for Skylake or later.
1. Thunderbolt 3, it's 40Gbps bidirectional via USB-C ports. Thunderbolt 3 is the first version to support external graphic cards, meaning you could add good desktop GPUs to play games on your rMBP, especially if you decide to just get the iGPU model.
2. Good chance dGPUs will be on 16nm FinFET, which means more powerful graphics that's more power efficient.
3. Potential chance of Apple adopting Intel's new XPoint storage subsystem, like the
Optane SSD as Intel/Mircon wants to start releasing it in Q1 of 2016.
4. Apple might adopt Intel Xeon Mobile Skylake for the high-end rMBP, latest leak shows the high-end model will include GT4 graphics with 128mb eDRAM. This is also the CPU that will support up to 64GB of memory, so we could see support for both 32 and 64GB of DDR4 memory. The latest lakes also shows there will be Skylake-H CPU at 25W as well, which could make rMBP more efficient at the base level.
5. Apple might adopt some new styling from the Macbook Retina, redesign the chassis to be lighter and thinner, especially since everything should be more energy efficient.
6. Assuming Apple does not switch to XPoint, the
upcoming switchover to 3D NAND later this year by all NAND companies should cause the capacity of SSDs to increase while remaining the same price. In other words, it is possible we might start seeing 15" start with 512GB and up to 2TB available.
Because of these changes coming in 2016 and the fact you won't see a major change in the new 2015 model, I'd suggest waiting to see if 2016 will be the one you want.
By the way, the upcoming El Capitan update will improve the performance on rMBP, making it feel much faster and thus the wait will be more tolerable. I definitely feel the difference with El Cap on my rMBP.
