Returned a couple of weeks ago from two weeks in Florida. The she-boss and I brought back 24K files (the vast majority will never see the light of day); shooting 4-6 hours/day in most opportune spots yielding a lot of time mashing the shutter.
When I look at the 'keepers' I see the vast majority are at 400mm or 400mm + 1.4tc.
I took a 500mm and it would do just fine until I'd tire wielding it...maybe it was the venues we visited that were just made for 400mm? Only one place so far, Conowingo, that I've really felt 500 is needed and honestly at Conowingo 500mm is the minimum entry the times I've been there.
Point? For me elation that Sony would bring out the 100-400. My hope of course is that it performs as well as the gold standard zoom + TC the EOS 100-400ii and 1.4TCiii. Another point- that wildlife photographers can do fine with a 100-400mm if it performs, and even better if it can with the TC. Sports (i.e. football, soccer, tennis,etc.) I know nothing about

If so then I'm in.
Owning a6000, a6300, A7Rii and a99ii I have no doubts about the prowess to be of the alpha9. I've seen enough in reviews/you-tubes to be completely confident in this new body-heck I've done respectable BIF work with even the A7Rii and adatped EOS lenses.
What I'm not sure about is the combo of the 100-400 with TC. That is to be seen. On this trip to Florida I shot mainly with a 1DXii but also enjoyed the a99ii and an Oly M1ii. Each has it's own set of blessings. My favorite of the three was the Olympus. Why? Weight and age.
If the alpha9 with the zoom and tc mentioned are up to EOS standards of IQ then my days with Canon will draw to an end (I will keep my EOS M5 however, it works a charm with the 400/5.6 and is just a super concise and light wildlife kit). Canon has been very very good to me over the years and I'm sure there are many things about the platform I'll miss. Weight will not be one of them.
Here's an album link to a couple of the Florida keepers-
https://www.flickr.com/photos/medicineman4040/albums/72157680274375832
and here's a link to a BIF album which is mostly via EOS kit and again at 400mm with or without 1.4tc-
https://www.flickr.com/photos/medicineman4040/albums/72157662637724813