I guess the assumption is based on the large amount of money handed over at the time of purchase.
That gets you whatever was sold to you at the time : Hardware and maybe 3 years of Applecare. You and me probably realise this, but a few people buying Mac Pros don't seem to be the typical enterprise/business buyer. More of a consumer/prosumer group that thinks updates just come freely and expect them.
Wait till they get to deal with real support contracts, they'll find out quick enough what EOS and EOL mean.
I just had a look and assuming one had purchased a Mac Pro in 2006 (the very first model) then it would be usable for at least 6 years which is a pretty good life for even a high end model
But you'd be a recurring Apple customer on the basis of Applecare only for 3 years, not 6 years. If you want 6 years of support, Apple doesn't offer it. Support can include phone, onsite and things like updates. That the machine is getting updates 6 years later is just gravy by Apple.
What people consider a normal unfortunately has no bearing with what a vendor will provide you as far as updates and support goes unfortunately in this industry. Apple is especially fast at ending support on software/hardware, that's just reality.
After all these years, why would people expect different from them ? Because they have a "Think different" slogan ? If anything, Apple thinks different by ending legacy support quickly and moving on to the next best thing very fast.
- I don't know many professionals with a high end workstation (one also assumes that performance is important based on such a high end performance) would keep something for that long.
We have some servers with HP under contract that are over 10 years old, but HP isn't exactly Apple. I will agree with you though, if you can do your work on a 6 year old Mac Pro, maybe you don't need a Mac Pro to begin with.
I do hope that Mountain Lion closes the 32bit chapter of Apples business through the removal of 32bit support from the kernel is the start of a long cycle of support due to a simpler level of support required not that 32bit compatibility being jettisoned.
People hoped that the switch to Intel would bring longer support intervals, and look what just happened with Moutain Lion.
If anything, with yearly OS X releases, I'd say start expecting that as soon as Applecare runs out on you, the next OS is off limit for your hardware. The reason why will probably be completely random. 32 bit right now is the excuse.