CHG_Canon, while not a Sony user, was certainly not precluded from answering since the OP also discussed part of their conundrum with Canon products. Well thought out and his final thought was relevant regardless of what one shoots.
Clearly you have no problem in answering any question since you have owned and are an expert in just about all major camera manufacturers. In fact, one can only be impressed with your stable of equipment..especially since you never let anyone forget.
And let us not forget that all of these professional organizations that use certain manufacturers, such as Sony, mean that all of us ignorant enthusiasts who choose to use anything else...are ignorant!
Thank you sincerely for pointing this out.
LoL, LoL, Cheers

I still shoot my a9, there is basically only cosmetic differences between an a9 and a9II, both share the same guts.
I also shoot the a1. The a9 does not have the eye focus of the a1 for birds but still I get almost a 99% keep rate on birds in flight with the a9. I like the vertical grip to make the camera larger for my hands, especially with longer lenes.
I notice hardly no difference in focusing speeds between my a1 and a9, they are both very fast. The a9 does not have a shutter like the one on the a1 when turning the camera off. So be careful changing lenses outside as your sensor is exposed on the a9.
Again, I still shoot wildlife with my a9, I find it still very relevant in the field. It would be a big bucks savings from the a1.
I will be selling one of my three a9's in May 2024. I will list it here first, I have the vertical grip for it and all boxes.

Canon limited? What??

Via the EF to RF adapter, the largest catalog in the history of photography is adaptable, as well as numerous other SLR-style lenses from multiple non-Canon mounts. Even Canon's own legacy FD lenses can leverage the marvels of digital mirrorless technology.

Moreover, Canon is pumping out an RF catalog as fast as the designs can be completed, tested and patented. Granted, there's a price-premium for 'new' and 'best', something you see in all three major players.

If you think changing systems will make you a better photographer, well ... that's the UHH community in a nutshell. Hopefully, you can see the foolishness of this all too common sentiment.

I shoot with a Sony A7iv and a Sony A7Rlll is my backup. Your question about the A9 has to be in relation to the type of shooting you do. Many of the event shooters who I work with went to the A9 line for the speed of focus. I waited for the A7iv, because I was shooting with an A7lll, and I wanted the better focusing system and processor on the A7iv. But some Sony engineers told me that the A9ll can be upgraded to almost the equivalent of the A1, except of course it does not have a 51mp sensor. So it depends upon what your style is as to which model I would recommend.

I moved from the a7iii and a7Riv to the a9 because of the focus speed. The a9 and ii and iii are fast with different bells and whistles . I missed the resolution of the a7Riv so I now also have the A1 . The A9 is my back up but it is also my primary star photography and low light photography camera. The A9 has less noise at higher ISO. Is the a9 still viable ....Yes. The a7iii is fast but not as fast as the A9. But what about the a7iv. A used a9 is less $$$ than a new a7iv. If you shoot birds, animals, and sports, the A9 series is the way to go.

The a9 introduced a new much faster read out sensor which comes with a few quirks of the sort never mentioned by reviewers ... likely cuz they are not real users.

I worked my way around 2 or 3 quirks that I no longer specifically remember, except for the one thaz still bugging me too often to be forgotten. That one is the 25k ISO limit for the silent e-shutter. For me, silence and low light ISO are joined at the hip. Testing the imaging at 40k and 50k ISO (available *only* with the mechanical shutter) reveals that ISO range to be very usable. But, when I need 50k ISO I usually also need a silent camera :-(

Maybe thaz no problem to you. Sorry, but I dont recall those other quirks, so youll hafta encounter them on your own. IIRC, one of them prevented multiframe or burst function in some circumstances.

Since moving from Canon, Nikon and Leica after 40 years using them all, to Sony in 2017, I have owned Sony A6500, A7RII, A7RIII, A7III, A9, A7RIV, A7SIII, and currently A1, A7RV, A9III (yes today).

The Sony A9 was the first fullframe stacked sensor mirrorless camera with a then top 20 fps ability. It is still an excellent camera today.

IMHO I would get a good used A9II which had some improvements over the A9.

https://mirrorlesscomparison.com/preview/sony-a9-vs-a9-ii/

https://cameradecision.com/compare/Sony-Alpha-A9-II-vs-Sony-Alpha-A9

A9II are at bargain prices for good used ones right now, so that would be a great camera to start your Sony journey with.

And yes Sony's E-mount is the most widely used ,and the most widely produced mirrorless-designed lens mount in the world, no other mirrorless lens mount comes even close, none and that includes Canon RF.

E-mount has over 200 quality E-mount choices and growing every day from Sony, Sigma, Tamron, Zeiss, Samyang/Rokinon, Tokina, Viltrox, Laowa, Meike and more.

Sony and Sigma both are about to announce some more great new E-mount lenses on Feb. 21st.

https://en.wikipedia.org/wiki/List_of_Sony_E-mount_lenses

https://en.wikipedia.org/wiki/List_of_third-party_Sony_E-mount_lenses

And five of the six top Worldwide News Services have moved exclusively to Sony mirrorless for all their staff photographers and staff videographers worldwide, including Associated Press; Canadian Press; all Gannett media including USA Today; UK's top News Service.

AFP (Agence France Presse) helped develop and test the Nikon Z9, so they use Z9/Z8 for their staffers.

None of these top News Services chose Canon mirrorless for their staffers, none, just a fact.


https://alphauniverse.com/stories/why-the-associated-press-just-switched-to-sony/

https://petapixel.com/2021/11/17/sony-is-now-the-exclusive-camera-provider-for-gannett-and-usa-today/

https://www.dpreview.com/news/4545693607/the-uk-largest-news-agency-partners-with-sony

https://petapixel.com/2022/01/31/canadas-largest-news-organization-moves-exclusively-to-sony-cameras/#:~:text=Canada's%20Largest%20News%20Agency%20Moves%20Exclusively%20to%20Sony%20Cameras,-Jan%2031%2C%202022&text=The%20Canadian%20Press%2C%20the%20largest,provider%20for%20the%20media%20company.

https://petapixel.com/2022/06/09/how-pro-photographers-helped-make-the-z9-from-prototype-to-flagship/

Sony leads in overall mirrorless sales and fullframe mirrorless sales for many good reasons. Latest BCN and CIPA numbers show that.

Cheers and best to you.

I do mostly fast action wildlife and birds. A friend of mine has the A7RIV and the A9. For fast/accurate AF and FPS the A9 wins for him. I recently picked up the original A9 for $1350 on sale from KEH. I had no e-mount lenses - only A-mount and Canon EF - so I am adapting. Right now I am using 3 adapters to cover most of my lenses and it is working fairly well. I am limited to around 12 FPS or less with adapter and most lenses. I am slowly picking up e-mount lenses - like the Tokina 20mm f2.

Below is a duck shot I did with the A9 and Sigma MC11 with Canon 400 5.6L and Canon 1.4X III. (560mm f8) - I was SHOCKED to see it locked on and held the duck at such great distance (small subject) and back round ! Image no.2 is a crop of full frame no.1 !
.