The advantages of BMG with mm wave 5G signals has already been thoroughly documented here. No one is getting true mm wave 5G yet in the US. They think its great because it is much better than the crappy 4G they have been using. It has also been stated by Li himself, that his BMG will be (or already is) cheaper than stainless. So a better product at a cheaper price, allowing Apple to increase their almighty profit margin (which is all they care about). So yes, if Li can put the infrastructure in place by the time mm wave is truly rolled out here in the US, then Apple will be happy to use this "new" material and increase profits with a better product.
And yes, it will likely be something other than phones where Apple first uses BMG. The airpods studio and/or airtags would be a logical first step in BMG use. Keep an eye on supply chain rumors...
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