Good question! Speaking in legal terms, not popular usage, generally the problem being addressed is not "you have too much market share," but rather, "you used your dominant position in one market to then dominate a different market." To stretch your analogy, it'd be like your pro boxer entering an amateur tournament and taking all the prize money. In the case of Microsoft's IE bundling, they were using their dominant position in the Operating System market to become dominant in the Web Browser market, by bundling IE with the OS and providing specific OS integration hooks which were unavailable to other browsers.
> If a competitor has a superior product, then why not just leave it up to the consumers to decide if it lives or dies.
The trouble comes when the entity in the dominant position squashes competition unfairly. In the IE case, Microsoft included a browser with every copy of the OS, so consumers had to overcome hurdles to even try a competitor's browser. Additionally, as I mentioned above, Microsoft provided secret hooks into the OS that were unavailable to other browsers, so it was impossible for a competitor to create a superior product. Those actions limited fair competition in the browser space, which usually results in worse outcomes for consumers or the market as a whole, hence the laws forbidding monopoly abuse.
Hope that helps give some idea of what happened there. Of course, you can read much more detail about the case & anti-trust law in general elsewhere.
> If a competitor has a superior product, then why not just leave it up to the consumers to decide if it lives or dies.
The trouble comes when the entity in the dominant position squashes competition unfairly. In the IE case, Microsoft included a browser with every copy of the OS, so consumers had to overcome hurdles to even try a competitor's browser. Additionally, as I mentioned above, Microsoft provided secret hooks into the OS that were unavailable to other browsers, so it was impossible for a competitor to create a superior product. Those actions limited fair competition in the browser space, which usually results in worse outcomes for consumers or the market as a whole, hence the laws forbidding monopoly abuse.
Hope that helps give some idea of what happened there. Of course, you can read much more detail about the case & anti-trust law in general elsewhere.