S9 is pretty old in term of ASIC. The current ROI is really high if you find one of those. They hardly make 10$ per days.
You must be really early in the game when it comes to ASIC miners...
S9's can't get old unless a better version comes out. S9's ROI time is
low long because bitcoin's price is not high enough.
Don't look at other mining hardware while making comparisons. Nobody would buy L3+'s If ltc was still 4$. Same goes for eth and gpu mining.
You'll never know when will btc take its market dominance back. What'll happen to those alts then?