You forgot the rest of the quote from your 3rd source:
"Courts look at the firm's market share, but typically do not find monopoly power if the firm (or a group of firms acting in concert) has less than 50 percent of the sales of a particular product or service within a certain geographic area. Some courts have required much higher percentages."
Most anti-trust cases have historically been against companies with 90+% market share: Standard Oil, Bell Telephone, Microsoft. I'd love to see an example of a company that was found to have a monopoly with only 60% market share.
In fact I think Standard Oil wasn't far above that by the time action was taken. They were also cutting prices and modernizing heavily.
To get much beyond that I think you need some special advantage like govt protection. I suspect the patent system, which literally guarantees monopolies to companies in return for a fixed fee, creates a big advantage for the leaders in tech markets. Hence modern leaders last a lot longer with more market share (and way higher margins) than they should.
"Courts look at the firm's market share, but typically do not find monopoly power if the firm (or a group of firms acting in concert) has less than 50 percent of the sales of a particular product or service within a certain geographic area. Some courts have required much higher percentages."
Most anti-trust cases have historically been against companies with 90+% market share: Standard Oil, Bell Telephone, Microsoft. I'd love to see an example of a company that was found to have a monopoly with only 60% market share.