Re: Objecting to "Traditional Respect"
These are likely clumsy comparisons, but here goes...
Shaking hands: if I extend my hand to shake yours, but you do not return the gesture, most people would think this rude and impolite and a lack of respect.
Saying hello and making eye contact: if I say "hello" and you either do not return the greeting or do not look at me (or worse, neither), most people would think this rude and impolite and a lack of respect.
The sitting seiza and performing rei can, in my mind, be as simple as the above. No more, no less. I believe they show genuine gratitude and thanks, but they do not have to. Basically it can contain what the person doing it wants it to contain.
First, however, is if your sensei would appreciate these opinions from you, or not. Only you can answer this question.
As to why or how your sensei has reached their current rank and now is becoming uncomfortable with the practice of bowing, we can only guess.
Ahmad's post is very good, as is Drew's.