Solar isn't a magic wand. It has costs and limitations, just like any other power source. If fusion is cheaper and more versatile, then it will displace solar.
However. . . Honestly, I think it's kind of stupid to pit solar and nuclear fusion against one another. Of all the possible replacements for fossil fuels, the only ones that seem to be truly scalable enough to fill our global energy are nuclear (fission or fusion) and solar. (And maybe possibly geothermal, though that's not really proven.) Solar and fusion have very different profiles, and will almost certainly prove to be complimentary technologies rather than head-to-head rivals.
Nuclear isn't electricity... nuclear is heat... that heat must be converted into electricity... that conversion costs $$$. If solar BOS drops below ~$1/w it will be cheaper to turn sunlight into electricity with photovoltaics than heat into electricity with a thermal plant. There's a reason that Ivanpah will likely be the last thermal solar plant.
The only type of nuclear that will be able to compete will be nuclear that isn't thermal... a technology that currently only exists on paper.
Here's an interesting article outlining why solar + wind and generators costing >$5/w generally don't mix...
When you need a generator to fill in power gaps... it's gotta be cheap.