Making gas unaffordable makes large parts of the planet uninhabitable in the short term.
We could ramp up public transport very quickly given the will to.
Not in the places that would become uninhabitable without gas.
You did say large parts of the planet, what large parts of the planet couldn't adopt public transport?
I didn't say that, but all parts of the planet that are too sparsely populated and have poor infrastructure. There is little point in having a bus if there are only a handful of people per square kilometer. There are large parts of the planet that don't have a reliable power grid either.
IMHO we should just abandon those areas and give them back to nature.
Power is actually pretty easy to solve compared to transportation. You can reliably generate either wind, water, or solar anywhere on the planet and store it locally. You're right though that centralizing populations tends to be the most efficient use of resources.