We could be in for turbulent times, if it works.
This would be particularly exciting. Ships, trains and physically large forms of transport (maybe even aircraft/spacecraft?!?) could have fusion reactors built in.
Electric cars charged by cheap power from local, neighbourhood fusion reactors. The power grid is decentralised with reliable baseload power sources, that are clean, reliable (no intermittent problems with weather with renewables) and safe. Power sources can be located where they are needed, whether it's in a basement, on a roof of an urban building or on Mars.
The holy grail that you're thinking of is aneutronic fusion, usually deuterium-helium3 or deuterium-lithium6. He3 is super rare though, so a D-He3 reactor using it might need to get the He3 from a bigger fusion reactor using D-D fusion, which produces it (and that pesky neutron).
Even better still might be proton-boron fusion; it needs temperatures an order of magnitude higher than D-D fusion (and magnetic confinement two or three orders of magnitude stronger), but produces far fewer neutrons (there are fewer undesirable side reactions). Alas, this route will produce four orders of magnitude less energy than the much simpler D-D reaction.
Given the scale of the engineering problems, we could even end up harnessing fusion power by building dyson spheres; a star might be the only feasible, stable way to build a fusion reactor. Hopefully our universe was set on an easier difficulty setting than that when it was instantiated.
Finally a good reason to go to the moon!
Furthermore many oil rich countries have shown much more interest in using their wealth to increase the wealth and power of the elite and to oppress and control the population at large. How will they react when they face a very real risk of losing power? How will the oppressed people react when they start seeing the wealth (and the power that comes with it) slipping out of the hands of the elite?
Enough to counter the fact that Russia gets 52% of its federal revenue from oil and gas?
I find it hard to believe that making desalination a little cheaper, or transport a little less expensive will even come close to counter balancing those sort of percentages.
And even if it did, the relative importance of nations will change. Nations that were important because they had fossil fuels will become less important, and nations that were held back by lack of resources could become more powerful.
Those sorts of changes in power balances would cause a lot of turbulence.
The massive leap forward that goes with any new breakthrough in energy supply or cost, is without question worth the potentially negative effects those countries will suffer. Saudi Arabia can find other ways to build economy, as so many other countries that lack oil have to do.
Russia is the only one on the list that worries me, because of what a truly destabilized Russia might cause.
Venezuela might even benefit from their oil system going to zero. It'll force their leadership and people to move to a functional political system, and away from the oil welfare bribery that Chavez used which has crippled the nation.
Really? I thought salt water desalination is a rather difficult and expensive thing to do. Can you cite support for this statement?
Having to get serious about building open modern secular self-sustaining economies would not be a bad influence on that region either.