Not pick on any one person but I see this a lot: anyone angling for a used Mac Pro 7.1 in 2-3 years is going to be sorely disappointed. The high cost ensures few units sold to end up on the market and with the upgradability makes it doubtful many users, even in high budget firms will be letting them go.
I think there is an assumption that Apple will roll out a very substantive upgrade within 24 months. Time will tell. Based on Apple's track record over the last 9-10 years, that wouldn't be very probable. If Apple has decided not to go back into deep Rip van Winkle mode then 3 years is probably a safer bet.
There are also a fair number of folks who will put these new Mac Pro on a 3 years (or less) depreciation schedule. There is another set on 3 year leases. (
Apple leasing has 2-3 years cycles ). If there is nothing newer to jump to then a higher percentage of folks will just buy out what they are leasing. However, if there is something new and the business has high rates of new cash flow (and very substantive profits ) probably will just option into a new equipment rather than take the system. ( or put more older/failover systems into retirement. So may need another year or so for it kick start into the pipeline. )
It is correct that they won't be dumping in overwhelmingly large numbers. However, the trickle of used should be fully pipelined by 3 years out. The numbers are likely to be low enough that the price discount for high quality used will be high ( unless can find several sellers in a fixed area who have to dump it for huge loss. ). The high budget, high profit shops probably would start to let go of some of these for substantively better equipment.
I think if Apple sells enough over the next 10 months then probably will get an update within 2 years for a couple of factors. One, in order to repair their reputation here they are actually going to have to "do something". There are going to be a decent number of "don't buy version 1 " buyers piled on top of the "well Apple is just going to flake again" buyers still hunkered down on older systems.
Second, this Mac Pro is coming at a competitive inflection point. The competition between AMD and Intel is only going to ramp higher in 2020. By 2021 if Apple is stilling standing there like a lost deer in the headlights, they simply are just going to get run over by a big truck. Even the changes coming to the mid-level systems at that point are going to creep well into the range offering here. Apple has also stumbled into a very much "dead ender" chipset (the priority right now is shipping something in 2019. Waiting yet another year isn't a viable long term option. )
If the Mac Pro first years sales underwhelm Apple's expectations then they'll just milk this system like a cash cow and go back into Rip van Winkle mode. In that context, the trickle will be even smaller which would mean even less of a discount. So the discount hunters would likely be more disappointed. There still would be some but a decent portion of that is folks jumping off the Mac platform (or official Mac Platform. Apple may have fertilized the hackintosh market way too much at that point with bozo moves to create a substantive problem in many locations. ).
I suspect Apple is looking to sell about 40K a year of these. Even if the run rate drops in half each year. 40 + 20 + 10 that is still a total of 70K for 3 years. ( if stays constant then 120K ). Even only 5% "get off" the Mac Pro ride that would still be 3.5-6K systems floating out into the used market. The high cost of the systems will also help cluster those systems so the world wide dilution won't be as high as other Apple products. The deep bargain hunters are also a bit "louder" on forums than numerous.