I imagine MySpace were saying something similar for a couple of years as Facebook started really taking off, and Yahoo probably thought no new search engines were a threat a year or two before Google arrived.
The Internet is a funny place. A lot of Internet users move in herds, which means a disruptive newcomer can grow very fast if the whole viral networking effect kicks in, but usually when that happens one or more established players must fall to create the space.
On the other hand, MySpace was not hooked on so many and so important portions of one's Internet life, nor did they have the advertising upper hand that Google has.
As for the Eternal September... I used to think it had been just a momentary problem that led to the unfortunate demise of USENET. I am starting to wonder if it wasn't, in fact, a first symptom. Programmers from my generation ought to have been so happy to see this happening -- the WWW available to everyone, connecting everyone. If only it weren't for the wrong reasons. The taste of my dream turned out to be a lot more bitter.
As for the Eternal September... I used to be able to talk to people on the internet, with threads, groups, and killfiles, using the software of my choice. All those moments will be lost in time, like tears in rain. Time to die.
We are still a good five or six years away from the point where Google would have to worry about alienating the users whose data they sell, and with the decreasing amount of Internet education most users get before starting to surf the web, the distance from that point is increasing. They thrive on the eternal September.