I use the bare lens mainly for indoor events (concerts and other performances), and I use it with the 1.4x mainly for field sports. It does take the 2x TC well, though I don't use that combo much. After getting the 100-300/2.8, I find that I am using the RF 70-200/2.8 much less. The 100-300 is a great pairing with the 24-105/2.8 for both indoor and outdoor events/sports.

I do suspect that the price of the 200-500/4 will be in the $14-15K range, not considering tariffs (because who knows if or what those will be, if/when the lens launches).

For me, even with the 2x TC the 100-300 is on the short side for birds (where I'm usually at 840mm f/5.6). But for large animals, with the TCs that would be the ideal range.

Rumor is that prices will be going up 7-8% very soon (next week, apparently), and for a $9500 lens that's basically the cost of a week's rental.

Mainly, I think it comes down to your use cases and the frequency at which you'd use it. I used a 70-200/2.8 extensively for events for years (EF MkII then RF), mostly my kids' performances. As they moved into middle school and high school, the venues got bigger and I was finding that 200mm wasn't long enough. But since that was post-RF, the 300/2.8 II was hard to get (not impossible), and even availability aside I was reluctant to spend that much on an EF lens. I was waiting for an RF 300/2.8, and was not thrilled with the size of the 100-300/2.8 relative to the EF prime but the flexibility of a zoom is more than worth the tradeoff. In terms of frequency, since getting the lens I've taken 22% of my saved images with it.