The issue is not that Solar isn't useful in helping cut CO2 emissions and mitigating Global Climate Change. Solar is a useful part of the technology mix and can help. The problem is that activists and environmentalists are content that Solar is going to give us the technology we need or as you essentially say it will allow us to kick the can down the road and wait for better technology to provide a real longer term solution.
I believe based on the projections and climate modeling that the threat is more urgent than waiting a decade or two before actively pursuing contingencies based on existing technology. We need to cut CO2 emissions now. Not merely cut the growth of CO2 emissions.
I see it as incredibly dangerous for the world to tinker and hope that new technology will provide a real solution down the road when we have no idea when it could be available or how long it could take to implement.
We have a real solution that is low risk with minimal environmental impact right now in nuclear power which would give us hundreds of years to figure out longer term solutions with new technology before fuel sources became more scarce. And we are literally risking the fate of civilization because on one side people can't figure out how to make enough money from it like they do selling oil and gas and on the other side people have been fooled into thinking it is more dangerous than it really is.
I don't see much difference between denying the science of greenhouse gases causing climate change and denying the science that we need to cut our CO2 emissions significantly more than Solar alone can reasonably expected to do. Unless you cut CO2 emissions by a sufficient amount to really begin to stabilize CO2 levels in the next 20 to 30 years and forestall Global climate change then policy simply does not matter and you might as well just focus on shorter term issues and adapt as best you can when climate change occurs as it will inevitably.
At this point the easy, cheap solar is what we are doing first. The adoption curve isn't going to get easier. Solar alone just can't get us where we need to be and we can't just hope it will give us time to come up with something better when the science and economics says otherwise.
Saying Solar adoption will let us kick the can down the road makes you just as much of a climate denier as those chanting "drill baby drill". We can't afford to just let the next generation figure it out or die trying. There is nothing good about that level of ignorance. Great Solar, now how do we actually save civilization?