I think this answers your original questions of why anyone would want a faster MorphOS/OS4 or classic machine. There are apps that run on the Amiga that people really like or are used to using. many of them run just fine on the next Gen Amiga systems.
Octamed is not one of them but, there are many trackers for the PC that have many more features including VST support. Why not tell people to just use those? You like Octamed, maybe another user really likes ImageFX?. That program would fly on a G5 and only runs on Amiga's.
Well, that's a case of chalk and cheese. You are comparing very different tasks.
My A1200 runs OctaMED SS as a MIDI sequencer. First of all, OctaMED SS is my preferred tool as I have years of experience with it. There is, or was, a version for windows but frankly, I didn't like the interface and secondly I tend not to run windows. Now, MIDI sequencing is not an intensive task; the 68040 in that machine is already more than fast enough for the job, hence there is nothing to be gained from running it on a faster box. In fact, I did try using UAE but in the end, there was little gain and rigging up my existing MIDI gear to it was a PITA. So, my A1200 is totally up to the job I ask of it. When I'm making music on it using my preferred application, it's already as good as it needs to be and no amount of additional go faster stripes actually help it.
Secondly, my PC does factor into music production. Recording, mixing, post production and soft synthesis. All of which IO/compute bound tasks for which my A1200 is underpowered.
Unlike MIDI sequencing, raytracing, is very much a compute/memory bound task. It's great that blender exists for AmigaOS4/MOS and hopefully time will see it fine-tuned and optimised, but even by then, I don't see it competing with an SSE3 optimised build on a fast x86-64 machine with several GB of high speed memory, running parallel render tasks on a true SMP platform.
In short, it's horses for courses. If you start justifying your choice of hardware by choosing compute bound tasks, why pick underpowered hardware to run them on? As cheap as you can pick up a G5 for, it isn't going to compete with an x86 box you spent the same total on (assuming you factor in the cost of the OS, which if you pick linux, is nada).