It is sustainable, they just need to boost their numbers and market penetration like every other venture that regularly gets discussed on HN whether it be a music streaming service or ride sharing app etc:

"Recent reports estimate that those numbers have risen to over 65,000 residential customers and about 5,500 commercial customers so far in 2015.48 This represents about a 45 percent share of the local residential market for broadband, video, and telephone service. 49 Despite being available to all potential customers, only about eight percent – or fewer than 5,500 – of residential customers that purchase Internet access subscribe to its signature gigabit service.5"

This venture is only 5 years old(2011) and the bond has a maturity length of 25 years! Lastly only 162 million out of the bond issue was used to build out the fiber network, the rest was used to build a smart grid.

Incorrect, EPB fiber is sustainable as they make a profit after debt service. There is no subsidy from the power side.

Why couldn't they raise prices by a bit?

No need. EPB fiber is already squarely in the black.

Presumably, they set prices to maximize revenue--raising prices would decrease revenue by losing subscribers.

Disregarding what amazon_not said about the facts on the ground, your objection is an interesting one in theory.

I think in such a situation I would perhaps try to play on people's pride and make an emotional appeal.

And/or sell equity to customers. Some are happy to hold part of an enterprise they believe in, and will accept submarket returns. Come to think of it, a cooperative wouldn't be too different.

Not necessarily. It's entirely plausble that they set the price lower in order to boost the number of subscribers and thus their political constituency.