I would be interested to hear about how many times questions related to that topic on the CHUF and AHUF were replaced by TEM.
Having recently passed AHUF I would anecdotally comment that this is a bit of a stretch. There were 10 questions out of 40 that were based on TEM. Hardly enough to "replace" the core Human Performance syllabus material. Don't be alarmed, the Vestibular system is still very well represented in the exam.
I did not get any actual questions on the Somatogravic Illusion, however I was well studied and prepared from the texts I had read on its details. It is one topic out of probably all of the ATPL syllabus that is extremely valid and will stay burned into my mind ever time I get into an aircraft at night.
I wouldn't go pointing the finger at CASA quite yet. I think the TEM component to the syllabus is quite well balanced.
RIP