It's not "out of the blue",
There was and is no new tech compelling enough to disrupt S/MS from the fundamental x86 PC architecture they chose,
so it was predictable the next generation would be trivially backwards compatable, however "branded" (NEO, etc).
The question of timing boils down to what and when AMD can offer in performance/cost, where S/MS are in the same boat.
Both systems were never sold at loss, and bw-compatability means no disruption of platform, so it comes down to AMD.
I think there is less connection between S/MS and NX than you think.
NX would have launched independent of S/MS upgrading just because WiiU is stalling and N is worried about mobiles.
If anything the correlation is simply down to APU/Fabs providing the same performance/price benefit impetus.
NX is not about cross-gen compatability like NEO, it is about cross-segment compatability, more like iPhone/iPad.
Nintendo needed to go there because it's console segment is extremely underperforming,
and it's handheld segment is facing a tidal wave of competition from mobile, even if doing better vs. console side...
Merging the two platforms means they mutually reinforce each other for a stronger overall platform/ecosystem,
which doubly helps when launching a non-trivially BW compatable platform from scratch with no install base.
( assuming NX rumors are true, of course 
)
