As a EM1_II (with the 300mm f4 pro lens) and Canon R5 owner, I would strongly agree with your post, up until the point where you compared the M43 300mm f4 pro lens with a Canon 600F4 lens. I'm afraid you drank the Olympus marketing coolaid - they loved to say that over and over. But the truth is that a supreme quality M43 300mm f4 lens would be equivalent to a supreme quality Canon 600mm *f8* lens, in the image it produces (including the 75mm entrance pupil(aperture) size and amount of light delivered to the *entire* sensor) as well as its approximate size and weight. The only difference would be the Canon sensor would have 4 times the area and thus 4 times well depth and thus would be able to have 4 times less image noise if given a 4 times longer exposure, and it may (or may not) have the same # of pixels, but both of those are a difference in the sensor, not the lens.I have an Olympus camera. Other than its crazy menu system its quite a nice camera. Very compact. Of course the sensor is limited as its relatively small compared to full frame but its not bad if you can get the image right in camera. I think their lenses are superb. .
It's not the lenses that have caused the failure of the company.
They just tied too much faith in micro 4/3 sensor size that they were unable to improve. Getting into computational photography earlier might have helped but the sensor size was inevitably going to catch up.
In a camera itself the sensor only takes up a small portion of the camera and a full frame sensor is not that much bigger than micro 4/3 in physical dimensions. Fully frame cameras were always going to get smaller.
The Olympus OM-D E-M1 mark ii / iii are great cameras and very good with the 300mm F4. It's a very convenient weight and size.
(Olympus naming conventions have been stupid - you can't even easily describe to someone else which one you have - should have been Olympus E1, E5 etc)
I know many happy owners.
A Canon 600F4 is not easy to carry around. Even to find a bag to hold it wasn't easy. I have hand held it but its not easy.
Any Olympus EM-1 with a 300mm is very easy carry in comparison. Mobility is quite a useful characteristic in nature shooting.