Okay, I'll pick this out of all the replies.
I don't have a narrative. I just wanted to get the big lump of initial sales out of the way and see how they aligned after release hype.
That doesn't make any sense. This gen happened to feature really large number of early adopters, so there was a big initial spike of sales. Previous gens happened to have those same sales spread out over more months.
Ignoring the first several months of PS4/XB1 sales implicitly assumes that those early adopters were never going to but a current-gen console at all. In a more accurate counter-factual, you'd need to to take those early sales and distribute them over the first few months of whenever you choose to start your imaginary time series.