OVERKILL
$100 Site Donor 2021
You do not want a monopoly of any source.
If your whole grid is coal powered the coal industry / union can go on strike and gouge you (UK remember?) Same for oil, same for natural gas, same for nuclear. The more you diversify even for non environmental reason, the better for bargaining power.
You do need a way to stabilize the grid so they don't go duck curve though.
Not if you aren't in a market. Bruce nuclear and OPG both get paid the same no matter what demand looks like. Fixed-rates for the generators also allow for very long-term plannings as revenue is easy to predict.