The economics sub-discipline of economic geography was being developed at about the same time as Eternal September.
The key insight (one of the key insights) from that research is that as the absolute cost of transport goes down, previously insignificant differences in cost become important. This leads to to the development of "hubs" - centralization.
(Here we're talking about information transport, and the cost being time per bit.)
But as you say, at the time the tech world could never have believed that centralization was the default expectation, nor designed things to compensate.
The internet observation is an adaptation of the original work on goods trade to other transport forms. I forget where I first read it--sorry! Maybe someone like Clay Shirky, but not the man himself.
Additionally, would they have been listened to?
In the early days, the internet was seen as a massively playfield-leveling and decentralizing force ("the net interprets censorship as damage and routes around it"), not a massively centralizing one (Facebook is the world's only newspaper).
In a model where everything is decentralized and leveled , no player is big enough to worry about.