They would have had less backlash had they simply left the Mac Pro as it was.
Extremely unlikely. Less backlash if the whole Mac line up was updated twice before the Mac Pro moved at all? I don't buy that. For most Apple major products there are yearly updates. Go off the yearly track and folks are going to complain. Go two years off the track and folks are going to assume it is dead (or at best a zombie).
Three factors in play.
1. It is against company policy to pre-announce products. So if there was no "new" Mac Pro there would be no opening to even drop hints at a follow on Mac Pro. No updates means there would now be no "clarification" that there will be another Mac Pro in 2013.
Even if Apple completely broke with policy and announced that nothing coming for a year you would have had tons of threads whining about how come no speed bumps in the mean time. Those speed bumps, at least along the CPU dimension, are exactly what got delivered. There have been several threads with folks lamenting no incremental updates over the last 12 months.
Now that the incremental came there is still complaining.... well welcome to rumors sites. There will be folks complaining even if Apple updated to E5's and newest video cards. ( not enough memory, not enough slots , not enough power cables , not enough ..... )
Apple coming out and saying "we screwed up.. won't have anything for another year and don't even have a minor conciliatory offering" would send even more folks off. Because Apple could have done something. To have absolutely nothing would mean not even pulling solutions that work with almost zero effort out of Intel's product catalog.
2. Lion and Mountain Lion are putting more pressure on folks with vintage/legacy systems. No Rosetta means the PowerPC folks in deep denial need to move on. Mountain Lion leaving behind MP 1,1 (and probably 1,2 since that was more artificial model number inflation than really a new model ) Slightly better Westmere offerings make for a better upgrade target than the mid-2010 offerings. And there is only one dangling Nehalem 3500 offering left at this point.
Apple will still loose some of those people to Windows and Hackintoshes, but they will loose less with these updates. It isn't about totally negating the "backlash". It is about stopping the losses. Many of those folks are going to complain about having to move anyway. If give them "nothing" to move to they will just whine about how should have just back-ported to their ancient hardware. A slightly bigger gap between their ancient hardware and updated system will get more to move and be happy once get back to working and stop focusing on having to move.
3. System vendors are actually shipping new workstations with E5 . Salespeople from HP/Dell/etc were about to launch into a huge FUD campaign against Apple. They still probably are, but Apple doing and saying absolutely nothing would make their competitors efforts all the more effective.
It is one thing when Apple can hide behind "well HP's workstations are Westmere based too. ". It is quite another when the competition is one generation up on Apple's offerings. Or even worse, two generations up on the former configuration line up with two 3500 series offerings. Apple's single CPU package would pummeled on price and performance by E5 1600s alternatives if they sat still. For example the older "mid level" offering was 4 core 3500 series. Versus a 6 core E5 1600 that is a huge gap for anything that scales with cores. At least with a 6 core Westmere the core count is the same and just loosing just on micro-architectural updates.
If Apple decreases the performance gap so that the OS migration costs become a factor for more folks, they will keep more folks from defecting.