Absolutelly, for longtime distance future is guaranted for BSDs' & Linuxes' distros as sure.Proprietary OSes will available as their publishers. DOSes are almost proprietary OSes, and only one open source version is still active, but its development is so, so, so slow (my opinion)...DOS is good as I said for small bussines if customer use one or two programs and have no time to learn other OSes.And they are the best as you said for nostalgia, as ZX Spectrum emulator and nowday free or public domain games for it
Also there are so many fakes maybe more than any other OS, and in the most cases they are from Russia newer (and they are absolutely illegal) and older from East bloc (in some cases their legality is unknown because they are so good clones of Western famous DOSes).