By this metric, dropshipping is the best business ever then, and not just a zero-value-added activity for bropreneurs who are good at Instagram marketing.

Dropshippers are just outsourced marketeers for the producers of whatever it is they sell. There's nothing wrong with that, real world products in a competitive market benefit from being advertised and you might as well outsource that activity to people who are good at it (ie bropreneurs on instagram).

It's hardly the best business ever though. Dropshipping only works well in a low-competition world, but since its barrier to entry is so low (all you need is an instagram account) you can be assured that competition will pop up very quickly and all your profit will be competed away.

Dropshipping is a great business idea. Amazon's price fixing has made it a zero sum game

Thought good business was a positive sum game...

Positive sum for society and/or your customers, perhaps. Almost never for your competitors.

Dropshipping is not sustainable.

They have to advertise and price items. eBay does not. eBay's users do that for them, for free.

And yet amazon is worth more then 50 times as much.