The A9 afaik is not that big
It is when you add the grip, which is needed for sports photography to avoid running out of battery power too often.
True, the other smaller bodies don't stop working- but what the other smaller bodies are missing is top tech of the system. They either use the years old outdated 16MP, or no weather sealing, or 1-2 generations behind AF, etc.
Still, I'm happy using my ancient GX8 bodies and the GM5.
The issue I see is that in the mean time a bigger camera is coming out at the $3k tier (USD) and you have to wonder how the unique selling proposition of the system is sold and differentiated with the very capable mirrorless out there. Even a Sony A7III.
Size of lenses, weight of the total system. Have you ever wandered around at a sports event for 10 hours with 3 camera bodies and 5-6 lenses, total weight 10-20 kg?
Shooting sports in lower light stadiums- which camera will do better ISO, DR, etc.? Or in daylight- again... I dunno.
Full frame is better at low light, much better. A full time sports pro would need two systems, but the longest lenses are rarely needed indoor.
So does Nikon. I have heard so far ZERO considering other systems from the FF people that use Canon or Nikon- other than Sony.
Canon and Nikon share more than 75% of the ILC market. Sony has a 14% share, or at least that was what it looked like a year ago. Canon and Nikon don't need to sell to anybody but their own customers to achieve a much larger volume than Sony.
Sony seems to be doing that just fine actually. Oh, and what about that Fuji XT3...
I think Canikon are much more afraid of Fuji than of Sony. Fuji is an old name in the camera business and will probably remain there. They have also proven to be very dynamic and able to launch new, innovative products at a fast pace. If Sony closes down its ILC business tomorrow, it will hardly be noticable in their annual report.