I can see a trend which means Y+ supplants C for some, as it gets to the standard that C was originally in the 80s and early 90s, when it had seat pitch around 37-40″, slightly better meals and more recline. As C became more competitive all of those increased, pushing F to the margins for many.
Bear in mind the pressures on Y+ are market driven. Whilst VS recently improved its Y+ hard product, it still isn’t that much better than BA to threaten BA’s loads in that class.
JL and NH have the same effect on each other, whilst NZ hard product currently isn’t special, it faces pressure from QF which is slightly better, so NZ new Y+ will be a significant leap forward (and we can see CX and MH both now talking about Y+ no doubt because the spread of Y+ means both are missing out on an opportunity).
However, VK is right they key will be fares relative to C. As NZ has no F class, it is putting C as its premium level, and really only has SQ and CX as major high end one stop competition between NZ and Europe (and QF for the US), so can price accordingly. However, the size of its proposed new Y+ cabin (50 compared to 39 on 744s)on the forthcoming 77Ws indicates it intends to sell more seats in that class than ever before.
Y+ is the new C, C is the new F, except for a handful of airlines with serious top end traffic (as can be seen by QF restricting it to LHR and LAX to Australia).