I regularly talk to Chinese vendors selling robot parts who want to talk over WhatsApp. This is ultimately fine for me but it’s an example of a scenario where I don’t really want to give anything more personal than necessary.

Sure that makes sense to me, however I will say that if I was a vendor selling stuff internationally I probably would want (and even require, depending on export controls) some level of de-anonymization on the buyer's part. Presumably at the very least you need to provide a shipping address to receive the goods, which is no less identifying or difficult to spoof than a phone number.

I suppose. I am happy to give them my business contact info, but my whatsapp is my personal number.

Post something like a prom dress for sale online and observe the messages you get from weird perverted men.

Then ask yourself if they should have a sellers real phone number.

But why not just use an email address or something for that case? I get how it would be ideal (and myself desire) to have one messaging tool which works in all cases, but it doesn't seem like a massive failure specifically of Signal to not work for this usage. Certainly not one large enough to warrant the torrent of comments I see any time Signal gets mentioned on HN