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Earlier this year Google announced they were bringing Google Fiber here. Immediately all the ISPs announced that they, too, would be offering FTTH speeds. One company announced 300mbit, another rolled out 300mbit as part of a roadmap to gigabit, and the third actually rolled out gigabit well ahead of Google Fiber, which is still applying for permits.
I agree with you in principle that there is some margin below which companies will not invest in expensive capital expenditures. However there is also a margin above which companies will not invest in capital expenditures, because they have a sure thing going. The question is where those lines are.
The data from Austin show that in a market considered "exceptionally competitive" by American standards, companies are willing to invest an order of magnitude more in capital expenditures than they currently do just by threatening to create a lower-margin market. Google's work here, which so far consists of exactly 0 fiber installs, a landing page, and a contract with the city, has been so effective at transforming the market here that it's being argued by locals that it would be more efficient to forget about actually installing any fiber and they should just make announcements in one city after another and wait for the ISPs to do it for them.