There is no reason to believe that private capital used to convert coal to gas would be spent instead on renewables.
Additionally, 100% renewables is not viable today outside of specific locations with concentrated hydroelectric power.
There are many problems with using Game Theory terms in real world/pragmatic discussion, but yes, clearly among them is that I anticipated this pedantic a response. You are right, it is not exactly a textbook example of a zero sum game and we can't prove it is one. I use the terminology metaphorically and not mathematically.
The question I pointed out is "how much does it resemble one?"
I think that it is much closer to one than not, and I do think there is plenty of reason to suggest that many actors in private capital would prefer money be spent on natural gas over renewables due to vested interests. Vested interests by themselves are something that I have seen modeled as a zero sum game, in theory and divorced from greater context. As just one part of the whole mess of motivations, politics, and sociology in trying to address climate change and energy industry spending, does it mean the entire thing is reducible to zero sum? Of course it doesn't. Does it add evidence that are enough zero sum games and zero sum-equivalent games in play that we can worry metaphorically that the entire thing is one giant zero sum game? Yes, it does, and that's all I was saying.
A) What metric are we ordering "second best" by. If it's Carbon and/or Methane (and it should be in discussion of climate impact), natural gas is never our "second best solution" and absolutely a red herring or distraction.
B) This is especially true on the longer time horizon in both cases in both directions as being a worryingly zero-sum game, because 1) today's billion dollar investment is tomorrow's entrenched sunk costs, and 2) without some huge advances in carbon and/or methane capture we can't "take back" emissions and in long term climate control every molecule of carbon/methane we don't produce matters (not even just ASAP, but starting four decades ago when scientists first started warning everyone about this stuff).