What an attitude -- at $5000, it's a professional tool meant to do a job that pays. (many creative professionals can pay for their Mac Pros with the proceeds from a single job.) Apple doesn't really optimize for people who keep their equipment around a very long time because it usually makes very little sense for creative professionals (and a lot of other Mac Pro users) to keep their machines around for too long, especially if they've got a workload that makes them wait for the machine.
It's not like the early '90s where the Indigo/Indy/Indigo2 each lasted five to six years as a product and each ran the current version of IRIX for nearly a decade, which was mandatory because you could easily spend $40,000 in 1992-dollars on an Indigo.
That having been said, I have no doubt that the Mac Pro's hardware will last a very long time -- and if your needs would objectively have been met sufficiently by a plastic MacBook, then I have no doubt that (even if it doesn't always run current software) the Mac Pro will be fast enough to do what you want for the forseeable future.
Or, it could be like the TiBook I got in 2003, which was woefully insufficient for everything I wanted to do by mid-2005. Even though we're currently in kind of a software performance requirements plateau right now doesn't mean we'll always be, and you may develop some interest or professional need, which brings the Mac Pro to its knees.
Expandability is definitely nice. Outside of hard disks, how much of it have you used? How much of it do you expect will affect what operating systems it'll ultimately be able to run? Most of the time, there's not much in the way of a graphics cutoff in a system with swappable graphics that really affects what operating systems will run. At best, you can improve the performance of a supported operating system with a newer GPU, but you'll almost certainly not be able to make an unsupported future operating system work by putting in a newer GPU.
Plenty of people our age have put that much money or more into computers. 2009 was when I got my ThinkPad, which I dumped $2500 into easily. 2011 was when I got my virtualization server, into which I've put at least $3000-4000 so far. If you've got a nice job, having the expendable income to buy too-much computer isn't very surprising at all.
Will that machine run Tiger? If so, keeping that machine as a server for your vintage machines (presuming you can't install netatalk on something else, or find an early Mac Mini to run 10.4, or use ASIP or the built-in file sharing on an older PowerPC/OS9 Mac, or even find an old PC or virtual machine to run NT4/2kServer/Server2003) may be better than bothering with a seven year old version of Mac OS X on a machine that should be able to run 10.8 quite handily.