Both Canon and Nikon have made some great lenses for their mirrorless mounts, but popular manufacturer Sigma has yet to make anything for the R or Z mount. Sigma's CEO says that won't last forever, though.
It’s true that the photography market has been systematically shrinking for years — in keeping with more and more people simply using smartphones to make photos and videos. Less cameras sold also means less potential customers interested in buying a lens, which is why our short‑term strategy indeed involves a possibility of introducing lenses with new mounts — to such photography systems as Nikon Z or Canon EOS R.
Yamaki qualified the statement, however, mentioning that at the moment, the company is trying to catch up to demand for its current lineup of lenses and that that might delay its ability to develop newer lenses.
I think it is likely only a matter of time until we see Sigma lenses for Nikon and Canon mirrorless mounts, particularly since both companies are moving away from prioritizing their DSLR mounts. Certainly, third-party lens options will be welcomed by photographers.