Do you have a link for sources 1 and 2, perchance? Also, I don't think you amortized the interest? As you pay down th bond, the interest cost goes down, so you have more to pay towards the principal...
https://www.cga.ct.gov/2012/rpt/2012-R-0515.htm
At 4% interest, 83 000 subscribers and attributing none of the costs of the fiber network to the smart meter project the payback period is 10 years and 7 months. A far attribution of costs to the smart meter project would bring the payback period to under 10 years.