Demo innovations (i.e. grist for the future, not already in the aggregate data) lately have included robots to do installation, designs with reduced installation reqs. The US DOE Sunshot Initiative page has the details of their programs supporting BOS, easier permitting, and so forth, although they have some interest in spinning a positive picture.

Cheap panels does suggest risk of slowdown, but there's also some room to shift further tradeoffs in design, i.e. as cost of manufacturing and efficiency become less of an issue more effort will go into designing panels that work well with the BOS improvements.

and assuming free solar cells, will solar rule the world?

It won't dominate dark areas, or assume 100% load (without batteries that push back dominance later), or price out already built nuclear plants (or coal and gas plants, absent massive carbon taxes). The claim that we might build so much solar as to match today's world electrical output (but not the output of that future time) wouldn't be shocking to me, although my guess would be for it to take longer than Kurzweil predicts (there are more efficiency gains to be had, but efficiency gives you free BOS savings by letting you use fewer panels, and the other areas will have to step up, especially for the later parts of his prediction).