See, that's exactly where I was previously not as convinced. The numbers may point to that - but we can't say for any certainty that this higher selling rate comes from former loyal X360 users switching platform. At this point, it could just as well be that there are more PS3 users upgrading to PS4 (than X360 -> X1), or more new consumers purchasing a PS4. Or more consumers that previously owned both a X360 and a PS3 and may get both next gen consoles, but bought a PS4 first - or a combination of all of the above.
I disagree:
It's hard to see a reason why 360 owners
on a huge scale are holding off updating compared to their PS3 owning counterparts, this seems like pure conjecture to me - in the first year of a console's life sales are mostly to hardcore gamers and the platform faithful - the impatient ones who want the next big thing now; the 'wait and see how' buyers or the 'waiting for great games' buyers will come into play later.
From their respective last gen performances, you would expect XB1 to be clearly ahead of the PS4 at this point in time (what with the US being Xbox heartland and early sales being dominated by the hardcore/fanboys) and yet the situation, at just 9 months from launch is that the PS4 is not only level with the XB1
but substantially ahead.
I mean for your theory to be correct, this limited upgrading by the 360 owners has to be
incredibly widespread among the early adopters. So much so, that it would suggest dramatic disaffection with MS among its fanbase - and in such a situation, to me at least, the more plausible response from said disaffected 360 owners looking to buy a next gen console would be to jump ship to Sony rather than to sit on their hands and see what MS does (I mean it's not like the console is going to get more powerful as the gen goes on).
It's not like Sony has better games either at this point in time (thereby incentivising PS3 owners to upgrade) - in fact MS seems to be better serving its fanbase with two2 Forza titles and a Halo title coming out in the XB1's first year.
And as for sales to non-gamers I hardly think that'll be a large proportion of the volume, unlike last gen where a significant amount of the PS3's initial sales were to people looking for a low cost Bluray player the PS4 has no similar selling point (in fact the XB1 makes for a better media box with its TV capabilities - especially in the US) as other media boxes - Roku, Apple TV or the PS3/360 are cheaper and more fully featured.
The alternative hypothesis that 360 owners are switching to PS4 is also borne out by a plethora of anecdotal evidence, online sentiment, various gaming polls etc. Not to mention Sony's statements that a lot of PS4 owners never owned a PS3 last gen. And as you mention, the Destiny preorders also provide some circumstantial evidence of a 360 fanbase shifting across to PS4.
So I definitely think it is the most plausible explanation we have for the swing in the US.