Let's look at actual numbers rather than fantasy.
> Upfront Investment: $140 billion
It cost Verizon at least $26 billion,[1] to wire up 7 million subscribers, or about $3,700 per subscriber. Verizon's network is in dense northeastern/midatlantic states, so in reality much more than that once you start wiring up the rest of the country. Chattanooga's network was about $5,500 per subscriber.
> Yearly revenue per Subscriber: $70 month * 12 months/year * .97 operating margin = $815
The gross operating margin for Chattanooga's gigabit system is under 40%.[2] So $336 per year.
> 140 billion / 50 million subscribers / $815 revenue per subscriber per year = 3.4 years
$3,700 per subscriber / $336 revenue per subscriber per year = 11 years. You've broken even just in time for the next around of upgrades!
> Installing fiber in dense neighborhoods with a high subscriber rate should recoup very quickly
Verizon only averages about 33% uptake rate, and that's by avoiding low-income inner cities or sparse rural areas.
[1] The reports are $23 billion through 2010. Don't have records after that, but e.g. it spent $3 billion in NYC alone since then. The subscriber count is 2016.
[2] Verizon's margin is quite a bit less than that (single digits), but let's assume in this hypothetical that you're not maintaining a legacy POTS network and don't have a unionized workforce.