I own four copies of fs2004 at the moment. On the first one the disk #4 decided to stop working as did one of the others (couldn't read the disks for reinstalling stuff anymore). The second copy I bought is the one I am now using. And I have two brand new "in the box" copies waiting in the wings in case anything happens to the one I am using.
They were dirt cheap at CompUSA a the time I bought them....well after FSX came out. That is a good "safety strategy" if you find you are going to be tied to a piece of software for a long period. Now a new copy of FS2004 is likely a "rare" item...and will command a good price.
As you can see, I am planning on staying with fs2004 for some time into the future!

I figure it will be at least two more years before I can get fs2004 running to the level that I want it to, what with all the add-ons I'm using and at the smoothness I want (powerful new main FS machine needed).
Plus the simpit I use is configured around fs2004....so re-doing all THAT stuff will be a huge investment in time.
I haven't seen fs2004 in the stores in a LONG while.
best,
...........john

Intel i7 960 quad 3.2G LGA 1366, Asus P6X58D Premium, 750W Corsair, 6 gig 1600 DDR3, Spinpoint 1TB 720