I see three problems with this argument:
1. Both Fortis and Hydro One are in the distribution and transmission business; not the generation business.  They like investments where they can manage a regulated rate of return rather than the big risks of these generation projects.  

2. No company is going to invest in big generation projects like these without a long-term purchase agreement for the output.  This transfers much of the risk back to the public sector.  You can look at Ontario to see what happens if you do not get the pricing on these agreements right.

3. Properly run government utilities provide both a fair rate of return to their owners and low rates to their customers (the same).  There is a greater tendency for privately owned  utilities to sacrifice low rates in return for a greater financial return.

Private-public partnerships, bringing the rigor of private sector ownership to specific projects without selling the utility, is a more positive solution.