The article lists demand-side and supply-side substitution as 2 of the 3 key factors, which basically means 'if a monopolist changes the price in this product,

The article lists demand-side and supply-side substitution as 2 of the 3 key factors, which basically means 'if a monopolist changes the price in this product, how much does it affect supply or demand in the other product', and then says that products at exhibit substitution are in the same markets. The problem with these definitions as applied to the tech industry in particular is that oftentimes the 'price' the consumer pays is non-monetary. For example, should Google's dominance in the search industry preclude it from cross-promoting its other services? If both search and also the product it's cross-promoting are free, how should courts go about figuring out whether that's ok? Presumably Google is getting paid in data, and is turning that data into money via Ads, but the actual accounting of how much that data is worth is only something that Google has access to -- and then to see how this works for competitors, the court would also need the proprietary internal numbers for them too.