It's not sustainable. They're making less than 4% return on a network they issued 4.5% bonds to build. Ratepayers who have no choice of electric company are subsidizing the internet service.
"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.
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.