Microsoft has quoted 18 months of support for devices from their release date(this is when the phone was released, NOT when you got it). This was publicly said and even things on WPC about it when WP8 was announced.
*IF* Microsoft stands behind their statements, Every NEWER device will get WP9 if it comes within the next year (announcing and releasing is 2 different things). This as long as there is not a major need for hardware change and I would bet some features will not be available on upgraded devices, as normal.
With this being said, at least the quad core phones(1520/ICON/930/HTC one, etc) would fit in this 18 month time frame *IF* WP9 is released in 2015 or early 2016. So pretty much the odds are phones from this year should get it, first models of single core or dual core might be questionable at this point.
As a PR issue, sure, phones release in 18 months will get it and older ones might not, I don't think it would be a PR mess, as most people in the US are tied to the 2 year contract/new phone thing. If you have owned a Android device, you know buying some models your support does not even go for a year.
The biggest question at this point, as most things are rumor, when will be it be released, then we could find out when a phone was released and figure it out from that from there.
I see no reason why MS would do a kernel change but, if that is changed, then all bets are off...