Well if its just an improved CPU and GPU performance then I'll stick with this generation of the RMBP. I mean if you keep waiting for the next best thing then you could be waiting for an infinite time.
The advantages are not enough for me to cancel my order.
No doubt, if you need a computer now, just buy it. I've always been one to encourage that.
This may be semantics, or maybe I'm miss judging your meaning, but I would not make the metaphor of "continually waiting for the next best thing" to waiting for an infinite amount of time.
There may be people that actually do take that to heart and literally keep waiting, but I think for others, when they say "I'm going to wait for Haswell" or something, it's not necessarily right to criticize for that. There are times when there is a great advancement forward, which is what people are waiting for.
For example, moving from Sandy Bridge to Ivy Bridge, that is the same CPU and GPU improvement because it's the same architecture. However the move from Nehalem to Sandy Bridge or Sandy Bridge to Haswell is a generational increase.
Also, I simplified it greatly by only mentioning that there are CPU and GPU improvements. Sandy Bridge brought the iGPU on-die with the CPU.
In the GPU side, this year comes with a massive generational leap in performance because the Nvidia/AMD were stuck at 40 nm for the last two generations, and just this year moved to 28 nm along with a completely new architecture for both companies.
TL;DR - I agree with your thought process, but I think it's way simple to make the assumption that "if you wait for the next thing, why not wait for the one after that". There are times when the change is a massive leap from the minor improvements before it.