the PPC platform, as of September when Snow Leopard is released, will not support the PPC platform.
Uh... say what?
Leopard is basically on feature parity with Snow Leopard, at least in the UI.
This. GrandCentralDispatch don't mean jack to a single-core G5. And Leopard is a pretty awesome OS, so it's not like you're being stuck with Tiger, Panther, Cheetah, or whatever.
Mac Pro and Intel Xserve were announced on August 7th 2006, the latter became available in November 2006, which means people can still have valid AppleCare 3-year warranty on their machines while Apple is already cutting off their support OS-wise.
Cutting off their support? How? I've seen no indication that Apple is EOLing Leopard or Leopard Server, discontinuing availability of support for it, or even stopping selling it to those who want it. If there are PPC users out there who aren't ready to buy a new machine but are still running Tiger or earlier, I'm sure Apple will happily take their $129 for Leopard.
If you bought that last-rev Power Mac G5 in, say, September 2006, it either came with, or you immediately upgraded it to, 10.4.9. You then had the choice of sticking with 10.4 (last upgraded to 10.4.11 in late 2007) or, in October 2007 or later, going to 10.5, which is now on 10.5.7. So you've had an option for a major (paid) OS update, as well as about 9 incremental updates for fixes, security, improvements, etc.
And that's not even counting application upgrades. Yeah, Apple is making the new OS Intel-specific since they haven't sold anything non-Intel in 3 years, but they're still going to have to offer PPC builds of stuff for a LONG time - the G5's aren't exactly slow even by today's standards.
I don't know how all the obscure Linux distros can always cover 15 different architectures (stuff like IA-64 and SPARC included)
As a long-time Linux user, I can tell you the "coverage" of those weird architectures varies greatly. A lot of stuff that's available in package form for x86 isn't available that way for less popular platforms, so you'll spend a lot of time compiling it if you want it. And there's no guarantee it'll compile without tweaks on your part. Just sayin'.