I don't think your logic holds up when it comes to "iMac M1 is 11.5mm thin since 2021... I can see that becoming ~6mm with a OLED display." or any thickness changes.
Look how thin the display is on the M2 MacBook Air. I don't have a micrometer, but it's not the majority of the overall thickness. The aluminum is thicker around the border of the display than the actual display thickness. Probably to improve structural integrity. I suspect they could've made the base thinner if they used a keyboard with shorter key travel. We all know what happened when Apple did this the last time.
I don't think it's going to make a difference with the iMac because there's just no reason to make the iMac thinner than it already is. Even if they could make it half as thin, 6 mm isn't going to make any difference when it's sitting on a desk. It needs to have structural integrity plus airflow. If they gave it a thinner frame people would be bending it... I can imagine iMac bendgate.
I suspect that maybe in five years all displays will all be micro LED, but who knows. I guess it depends on what you're using the computer for as to if that means anything. I picked the MacBook Air even though some people that swear the display/ computer is completely unusable. I looked at the Pro and the Air when I went to the Apple store. I did think the Pro display was slightly better but it wasn't this huge difference. I was more concerned about light weight.
I don't think this is going to be something to be concerned with in the near future. Maybe the MacBook Pro will have an improved display but who would spend another 2k+ just for an upgraded display. I think this is one of those things that people will benefit from when their Mac reaches end of life and they upgrade. If next year they came out with a brand new MacBook Air that had a micro LED display I wouldn't get it. I was tempted with the 15" Air, but then realized I'm perfectly fine with what I have and it's just spending money to spend it