Well again, to be fair, you’re not sure whether such a design could fit in an r5 without some other compromise. The truth is, we don’t know. Maybe their design is better than Canon for some use cases. Maybe it will influence a future Canon design. People with their wallets will ultimately decide...At least Sigma show that high quality 4K 30p RAW would not overheat in smaller form-factor. And you don't use VF much while video recording. And about 24MP of Fp, R6's overheating situation is even worse than R5 with 20MP.
I don't know whether with adding passive grill, if design as good as Sigma, could R5 do 8K or 4k120p non-overheating, but 4K HQ non-overheating would be within reach. And even 8K or 4K120p would have longer recording time and much lower cooldown time.
Sigma has also delayed their FF foveon camera - again, we don’t know other than it didn’t meet their criteria. Maybe Canon thought theirs achieved their criteria. Purchasers must decide if it meets theirs.
Oh and re VF. Hmmm. For actual videographers, sure. But one might add they are likely to use an external monitor. For a hybrid camera with IBIS I wonder if the EVF might be easier to use than the rear screen. I haven’t tried yet, but I think maybe just like I use the evf for stills I might use it for video....