My guess, based on your analysis of the R5 internals, is that Canon intentionally neglected heat management in a bid to limit its appeal to video shooters. But they overshot the mark. (or maybe they only ever wanted hobbyist videographers to buy in)
Meanwhile, Sony capped the resolution of the A7SIII to limit its appeal to photogs and hybrid shooters.
Works for me though. I just want a low-light beast with good AF and 10-bit beefy codecs to ride on my Ronin-S all day