Don't know if this will make any sense, .... but think of it like this. Let's say you upgrade every 2 years.
Situation 1: You bought the Haswell rMBP at the end of 2013. The next time you upgrade will be at the end of 2015, which will be the Skylake version.
Situation 2: You buy the Broadwell/Maxwell rMBP at the end of 2014. The next time you will upgrade will be at the end of 2016, which let's say will have NVIDIA Pascal graphics.
Now let's look at the years.
End of 2015. If you are in situation 1, you will be happy and upgrade to a kickass Skylake rMBP. If you are in situation 2, you have to wait and use your "old Broad/Maxwell" rMBP for another year. So to be happy at the end of 2015, you should be in situation 1.
End of 2016. If you are in situation 1, you will be sad. You just got your Skylake rMBP a year ago, so no upgrading to the new awesome rMBP with Pascal graphics. Gotta stick with an old Skylake rMBP. If you are in situation 2, you are happy. You get to upgrade to the newest model, yay.
In situation 1 you will be happy in 2013, 2015, 2017, etc. Sad in 2014, 2016, 2018, etc (these are the years people in situation 2 are happy).
Conclusion, the waiting game makes no sense. The sooner you buy, the sooner you will upgrade again.
Keep in mind that the year over year changes usually aren't world-changing, for multiple reasons.