cfoesch, not sure what you are smoking, or if you are a coder at MS and have access to arcane knowledge.
The later. I've actually compiled Windows XP, Server 2003, and Vista.
XP/2K3, Vista/2K8, 7/2K8R2 are basically the same "generation" of NT.
Yes, those pairings are the accurate pairings of client and server generations... but that alone is almost as useless as it is to note that Windows 98 and Windows 2000 are the same "generation". Namely, in truth, they are just the client/server OS pairings, and it says nothing of how the code between the generations are related.
WinXP 32-bit and Server 2003 were developed separately from Windows 2000, and are built from different codebases. They are thus sister codelines. (Pedantic note: Windows XP 64-bit was (is) built from the Server 2003 codebase, that's why there were (are) so many compatibility issues... people are actually running 2k3, and not XP. They are slightly different operating systems, with unique quirks.)
This is different from the situation with Vista SP1+, and Server 2k8, as well as Windows 7 and Server 2k8 R2. Where the client and server OS are built out of the exact same codebase, and the only things that change are the binaries included, and the branding. They are not sister codelines, they are literally the exact same codeline.
(Pedantic note: the Vista RTM codebase had internal versions of what would be Server 2k8, at the time referred to as Longhorn Server, that were built along with the client, but none of these were ever released. Server 2k8 RTM is however the exact same codebase as Vista SP1.)
XP is not an "extinct line", it's part of the NT family and has nothing to do with Win9x "family wise".
Longhorn began development from the XP codebase, because it was going to be the next version of the client OS, and Microsoft had developed the client OS separately from the server OS for years.
However, when it became apparent that 64-bit was going to become common in the consumer market, they weighed extending the 32-bit only early Longhorn (and after extending the 2k3 codebase, they knew how much work it would take to work out the bugs), against throwing everything out and starting from scratch from an already 64-bit capable codebase (Server 2003 being already stable and established). They chose the later, and that is in part why Longhorn suddenly lost so many features that they had been boasting about for so long.
All of this was offset by the fact that Longhorn was now 64-bit capable. And while they did port some (perhaps even a lot) of the changes in the earlier Longhorn development into this new Longhorn, they did not carry over everything. This is why there were compatibility issues between Vista and XP... Vista is a descendant of Server 2003, and not Windows XP.
So, while Vista/2k8 and Win7/2k8R2 descend directly from the Server 2003 codebase, no product or codebase descends directly from the Windows XP codebase. Thus, Windows XP is an "extinct line". The fact that its cousins live on, is cold comfort though to an extinct line...