Especially after the producers of DS9 tried to steal ideas from their rival Babylon 5(a show that was more fun to watch).
Even if the theft was true, and that's a mighty big if, there's no evidence whatsoever that there is any correlation between the timing of any such alleged theft and the decline in ratings of DS9. Its decline in ratings has been discussed elsewhere in TrekBBS, for example in the thread of this post
. The decline appears indicative and entirely typical of a trend that encompasses DS9, VOY, and ENT together.