It depends on how the Justice department feels, to some extent. GM in the 50s and 60s actually kept an eye on their market share, and sometimes would kill/cutback projects or products that may have threatened going over a certain percentage (around 60%, IIRC.) At that time merely going over that number would have been enough to start an investigation.
Some would argue though that back then DOJ 'cared' more.
The summary is that the Chicago school of Economics promoted the idea that in a competitive market, predatory pricing is not possible. Thus, when you see an industry structure emerge (e.g. duopoly), you should assume that it is an efficient competitive outcome. After this, instead of focusing on market share percentages, antitrust authorities began to focus primarily on consumer harm (which is much more challenging to show in court e.g. for predatory pricing, you cannot only argue that your competitor has very low prices right now, you also have to prove that they intend to raise prices once they achieve monopoly power).
Jurisprudence also changed on the topic since then. In particular, Robert Bork published the legal theory of consumer welfare in 1978. SCOTUS essentially adapted that wholesale into case law starting with Reiter v. Sototone.
Given the current makeup of the court, it seems extremely unlikely that the justices would overturn this precedent.
Some would argue though that back then DOJ 'cared' more.