I think the B580 acheiving 4070 performance is a little farfetched, but even if it did, how much does that matter?
A 4070 today is under £500, a 4060Ti 16GB is £420, 4060Ti 8GB is £320. AMD's equivalent (in raster) is the 7800XT at £450, the 7700XT is £350. All OCUK prices, maybe you can pay less elsewhere.
Assuming the next gen of both NVIDIA and AMD improves price/performance by just one tier, then maybe the B580 will be OK. If NVIDIA is at £400-ish, AMD at £350, and Intel at £260, then it looks like a bargain (sort of).
However, if the B580 is 10-15% slower than the 4070 on average, which puts it closer to 4060Ti performance, then it's DOA, because better/cheaper cards are only 3 months away. Even the 4060Ti 8GB and 7700XT would only need to drop another 10%, before they're equal to (or better than) the B580.
Intel still have a lot of work to do