I like many would buy the Sigma 300-600 f4 and personally, the Sigma 135 f1.4 Art (I have the EF 135, RF 135 and Sigma EF 135 Art) and would pair it with my Sigma EF 105.
I wouldn't pay Canon's price for the RF 100-300 f2.8 before the tariff increase, as I have the Sigma EF 120-300 2.8 and I won't pay for the Canon RF 300-600 f???, as is would be a want and not a need. I have 2 of the EF 200-400 f4 1.4x, the EF 300 f2.8 and the EF 400 f4 DO and I refuse to pay over $11,000 for a lens, but at $6599 for the Sigma, I would buy that, provided it integrates well. Sony limits the AF speed on it, so I won't buy the Sony version. In fact, I was seriously considering the Lumix S1 II, or maybe the S1R II given the alliance and the fact that it is optimized for the Lumix.
I will hold off for now, to see if they release it for the RF mount, if it doesn't happen, then I will buy a Lumix down the line and begrudge Canon for it.
Oh, and I know a retired Canon USA exec, I never asked for any scoops, I didn't think it was appropriate, but was told, "Pay attention to Canon Rumors." So you might have more pull than you know