Streaming services pretty much top out at 80Mbps, but more typically are around 15≃20Mbps for even 4K content, so even if they straight quadrupled the bitrate for 8K content you'd only be hitting UHD BD rates.

I don't disagree that BD will not exist for an 8K market, but that's because physical media is being killed.

This isn't even getting into the actual mastered resolution of much of this content, which you're lucky if it's even in 4K, most stuff is still mastered in 2K.

That being said, UHD BD is at around 75 mbps. So 8K would be around 300 mbps,

I don't think multiplying by 4 is correct here. Even though it's 4 times as many pixels, this is compressed video we're talking about, the limiting factor is how many details are in motion. 1080p blurays are like 40mbps, 4k blurays are only double that bitrate with 4x as many pixels and 25% more color data (SDR8 vs HDR10).

It's a better codec which helps, but 8k could use h266. Also I think 4k blurays have less artifacting than 1080p blurays, so 4k blurays seem to have a surplus of bitrate relative to their content.

I think there exist bluray drives that can support up to 144mbps, so I don't think it's much of a stretch that we could make 8k blurays that look better than 4k blurays with existing tech and the h266 codec. But making people care about even more quality is another matter, if their eyes can't see anything finer than 4k anyways. Most people can't even tell the difference between 4k on streaming services vs 4k on disc.