David,
Thank you for taking the time to respond in such detail, very much appreciated. Also, my hats off to you for capturing moments in adverse weather conditions as shown, that is a big no "Thank you" for me....lol. I'm a fair weather photographer....
Off topic for a minute, is that part of your editorial assignment, or for your personal business, if I may ask?
I do not like exposing my equipment to the elements such as that, I would assume you had some form of rain protection for your gear while you were shooting?
Back to the lens, I hate a telescoping lens as much as the next person, I own the rf70-200, and used to own the ef100-400. As I mentioned I avoid rain at all costs, but for my shooting genre, dust is a huge factor. When I owned the 100-400 push/pull zoom, I would cringe as I heard it "breath" during zooming, and after, I would say a year later, internal dust became very obvious. On the RF 70-200, after two years of use during some pretty harsh shooting conditions times, I has yet to see signs of dust - but as you point out, seals wear and its likely to be only a matter if time.
Your concern for weather sealing has me rethinking this choice, after all of the great photographs I have seen taken with the 100-500, yours included. As most probably my last major lens purchase, I'm now back to leaning towards the 100-300 with teleconverters for addition reach.
Guess I will have to sell something to afford the purchase, I'm going to miss the wife and kids....lol
regards, Mark