Let's cut to the chase, a fusion reactor - which will have it's own set of environmental problems by the way - will depend wholly on a fission infrastructure for the forseeable future.

There are a lot of things that haven't been touched by the fusion reality (as opposed to the popular fusion fantasy.)   Among these are heat transfer (most of the energy is gamma radiation which does not interact that readily with matter to make heat) and the matter of tritium breeding, which is not only practically difficult but is also theoretically difficult.   And let's be clear, fusion reactors for the forseeable future will need tritium.   The total amount of tritium on this planet right now that is isolated is about 20 kg, and the vast majority of it is in Canada.   That isn't going to allow anyone to live forever in an energy nirvana.

Now, I really won't mind it if humanity had access to a some high energy neutrons from fusion reactors.   There's a lot you could do with 14 MeV.   I don't think it's going to happen in any time frame that will matter.

We are stuck with the tools we have, and we must choose between them immediately.

As for conservation, that's wonderful, but appeals to it are often parochial.   I have yet to meet a Westerner who waxes romantic about "conservation," who has ever begun to live a lifestyle consistent with, say, the Chadian per capita energy use.

It is great to talk about "conservation," if you are talking about buying a car, but "conservation," is not an issue when you aspire, someday, to have a light bulb of your own.   This is usually a surprise but "most people" don't have cars and computers.

We are not going to "conserve" our way out of this crisis.    There is not going to be a "two hundred years from now" until we fix what's happening right now