Norman,
Yes the track only does so many thousand miles, but how far are you planning on driving? You'll run out of money to pay for fuel before you'll run out of tracks. The rubber track will also wear out, how many miles will it last? No idea, but when it wears out you'll be looking for ordinary track to replace it. Small excavators don't drive around at 45mph and certainly don't do thousands of miles.
Sabres were mostly released with worn out track but most of the other CVRTs were released with pretty good track, I've seen Spartans with new track. The last 432 we had also had new track fitted. Although that's not important anymore, since there won't be any more CVRTs or 432s released for a long time. Any tracked vehicle you buy (unless you want something Russian or a Chieftain AVRE) will come from a private sale and who knows how well they've looked after it.
You're right, the bushes can wear out, but you can remove a link to regain the tension. Once so many links have been removed you need to replace the track. Track is avaliable, expensive, yes, but then how much does it cost to replace all 10 tyres on a GMC or a Reo? A set of CVRT track is probably the same price.
Everyone goes on about how impractical it is working on a tracked vehicle, maintenance costs, cost of track etc, but very few of them have owned one or worked on one. It's not that bad really!
Chris