Not all monopolies are illegal, although some are, but illegal isn't really the correct term anyway. They are simply undesirable, which is why competition laws are designed to prevent them from forming in the first place.
In cases where they form naturally steps are usually taken to either heavily regulate the monopolised industry to prevent abuse, convert it to a public service or break up the company.
Originally Posted by SuperZan
And now I've gone down the rabbit-hole.https://www.justice.gov/atr/competition-and-monopoly-single-firm-conduct-under-section-2-sherman-act-chapter-2
This is very interesting. I'm inclined to research it from a US PoV as I think that any real antitrust/monopoly intervention would have to be rooted in American courts to really put a hurting on Intel as that's their home-base.
It just gets more interesting the further you go. At this point though, I think that Intel would've already faced some kind of regulatory penalty as they meet a number of the criteria, which leads me back to thinking that the x86 market is not taken piecemeal but as a part of a larger industry including ARM and other semiconductor industry.
That's interesting, although I doubt how they define individual markets is specified very accurately. IIRC though AMD's last antitrust complaint against Intel was specifically for abusing it's position as the leading manufacturer of x86 chips, so I don't think the US courts have any issue with defining it as a distinct market.
I don't know much about US legislation, but usually in the EU an antitrust investigation (like the current one with Google) follows a complaint that the courts will investigate. I assume the market segment in question would be defined in the complaint, and the courts would decide whether the complaint was valid and what action (if any) would need to be taken.
I suspect the main reason they haven't done anything to date is simply because they still have a competitor, and as that bit of the Sherman act you quoted says "a high market share provides no reliable indication of the potential for rivals to supply market demand."