https://en.wikipedia.org/wiki/Relevant_market#Definition_and...
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.
Like, MS got hit for leveraging their 'OS market' monopoly to break into the 'browser market', even though they are both pieces of software.
If you draw distinct markets aggressively enough, then everything is its own market and many companies are monopolists in their own narrow niche.
Also, what if technological changes turn two originally separate markets into a unified entity -- should it be anticompetitive if a monopolist does that?