an iMac is not really a viable option at the moment, because there are less CPU cores available, which -although certainly not optimal working at the moment- is pretty essential in audio recording.
Sometimes the actual root cause motivations for audio (and other vertical segments ) "rules of thumb" get lost over time. "rules of thumb" typically have some assumptions weaved into them to render them more "simple" rather that clear as the root causes.
Since audio operates in the kHhz range and CPUs these days operate in the GHz range it is not so clear that a 4GHz four core of the same architecture is all that too much different from a 2GHz eight core. It is not so much the number of cores as how many timeslices can be given to a individual channel process that needs time on a core. If there is more time you don't necessarily need more cores.
This is similar to the audio rule of them assigning a storage audo stream to individual disks. That is more a rule to get around random access latencies for rotational disk with a seeking arm/head then necessarily a property of a storage disk. SSDs deal with random access with much lower (although could be lower for most ) latencies. Stream multiple relatively low bandwidth audio data sterams off a single 6Gb/s SSD isn't too hard.
The top end 27" iMac is a 3.4GHz , SATA III four core capable machine. That is better than top end 2006 Mac Pro ( older, slower memory architecture and I/O subsystem , 3.0Ghz quad core top end ). Those machines were viable to do audio work on back then for more than a few folks.
Audio's data rates are largely what they were 6 years ago and largely what they will be 6 years from now. The human ear isn't going to change significantly over that timespan.
Pragmatically the workloads largely go up by folks appealing to doing pragmatically larger ensembles than more processing to get to better music. A Mac Pro is more viable over a larger range of workloads, but minimal entry point for any viability at all is a stretch.
From a more personal perspective I dislike the fact that the iMac is not user servicable/ upgradable anymore.
the 27" is user upgradable. The is a small leap to say that the previous iMacs were user serviceable. A suction cups and dust issues glass/LCD gap placed it in the could be done, but probably out of the should be done for "average" iMac user.
That could mean terrible down time if we had to bring in the machine for repair.
It isn't going to work for everyone but...
If down time is critical having redundant parts for everything (i.e., a second computer) is essential.
2 * 2,500 = 5,000 ( 2 mac Pros )
3 * 1700 = 5,100 ( 3 iMacs )
For both could deploy 2. In the Mac Pro case all works fails over to just one Mac Pro. In the iMac case still have 2 after have sent the failed iMac off to be repaired.
The "Forumla One pit crew" component replacement times is largely root cause driven by the Mac being used as single point of failure. The most probable single point of failure are storage drives. If the drive is the most likely point of failure the easiest solution for that with the current iMac is not to put the drive inside the iMac.
Much of this boils down to "form" rather than "function". Studios , racks , or work areas have a 'box slot" where the Mac Pro goes now and there is a desire to have a box in there afterwards.
if Apple clearly told its customers it has nothing more in the pipeline regarding the MacPro, ...
Apple hasn't really said much one way or the other. If anything they have said hey
do have something in the pipeline for some time 2013. But to claim to say they don't have anything in motion at all ... there is no quote for that. Lack of comment is not a statement of empty pipeline.
Apple doesn't have custom configurations in the EU markets right now. However, "right now" is not a pipeline. Small quantity refurbs ( which would work if want 1-2 ) are out there in limited configurations.
The physical retail Apple stores have no inventory pipeline right now. But that is not the scope of what Apple sales nor the scope of available commercial sources for a Mac Pro in EU markets.