Truly new ideas are, say, 0.01% of all 'novel' ideas, with the rest being rehashing or recombinations. And even then, the truly new kernel of the truly new idea is some small fraction of the whole thing. Like Einstein's theories of relativity, they were truly fresh, but mostly had to recombine existing work in 'novel' ways.

So it's not there are zero new ideas, but so few compared to the whole of the intellectual domain, that we can say 'there are no new ideas under the sun' and be basically correct to within a small rounding error. And AI is sufficiently ingenious to serve the needs of the vast majority of humanity, even though we'll still need to find/cultivate/inspire some new genius humans if we want to have more truly new ideas in the future.