Why would, e.g., solar and chemical or physical storage be a stopgap? Why spend 20 years of building a fission reactor these days (other than for research, medical, or defense purposes) which also make awful targets in a conflict? Maybe just wait till fusion reactors are there.
Because the reactor will still run 20 years after that while the solar and storage will need to be replaced by then
Why would fusion reactors magically appear when the entire field of nuclear energy production is, in this scenario, essentially dead??