The 'flaw' with the extender is indeed a minus for this lens. Most of the time I could work around it as I was using the far end of the focal range anyhow. When the RF200-800 came out, I bought into it as a more suitable lens when walking through nature and see what birds or other subjects show up to be photographed. The wide range of focal length makes one realize a bit more how inconvenient that extender limitation is with the RF 100-500.
That being said, I still love the RF 100-500 and the compact size it has with the quality and range you get from it. It is easy enough to trow into the bag. I do sports, rugby especially, and was mostly using a Sigma 120-300 for its f2.8 on one camera and a second body with a 70-200 or 24-70. Two lensesI almost always take with me to the pitch. The 100-500 could be trown into the bag, in case 'my' team was having a poor match and most of the action was deep into the other half of the pitch. The extra 200 mm than helped to get some better shots of the team.
Recently I bought an old EF 400 IS USM L f2.8 which is almost 5,5 kg. That in combination with a 70-200 is now my main setup for rugby, so the 100-500 gets less use. I loaned it to a starting rugby photographer so she could try it. She shoots with a single camera (R6 ii) and for her the 100-500 range is very useful and she really liked using that lens.
If Canon came out with an internal zoom RF 100-500 I don't think I would change from the current lens. The compactness is just a big plus for me. (As for the RF 70-200). If you take some care for your gear, I do not see the external zoom being a real problem. The extender compatibility may be better for the newer version, but since I have the 200-800, it is less of importance to me. Even though the RF 200-800 is not to big and heavy for what you get, it is still a big lens that does not fit a simple sling bag that one would take during a ride on the bike. The smallish RF 100-500 would.