[aidenn0]

Here's an example:

Let's imagine a hypothetical world where Company A owns 80% of the smartphone market and the rest is divided between the also-rans.

Now imagine that app developers can't profitably make a 1st rate app without getting the income from company A's app store (But can port the app to other OSes for extra income). If company A starts to use this situation to make it harder to port apps to other companies OSes via strongarm tactics, then company A is abusing its monopoly position of being the only profitable way to make an app.

Today's world is nothing like that, but it's an example of how near-total marketshare is not necessary for anti-trust to come into play. There is a certain point at which you have a large enough market share, that certain other players in the market must deal with you to get the volume needed to compete, particularly in high-volume, high-upfront cost industries (which many types of software qualify as).