TNG had a rough start too because of its earlier seasons, or so I've heard. I've also heard that it really took off in season 4 and onward. Do you know why, or at least have an idea, this happened for TNG but not DS9 which had a wonderful season 3-6 (imo)?
TV was changing during the time TNG and DS9 were on, and is still changing. Audiences are lower across the board. More competition means everything is being forced into a niche audience. Star Trek will never have the mass audience of TNG ever again. DS9's niche approach - smaller audience, less accessible material - is the right approach for all space opera now, and for sci fi in general.
Serialzed shows do great and dark shows do great - on cable, which is the only place for a space opera series nowadays. Broadcast is spiraling the drain except for CBS, which is all cop shows, sitcoms and reality TV - the lowest common denomenator.
In addition to cable, streaming services like Netflix and Amazon are other good places for original Star Trek. Moonves already is on friendly terms with Netflix. Something may be brewing there...
The handy ratings chart demonstrates that the approach is not what drove ratings down. Serialized or episodic, dark or fluffy, it made no difference to the trend line. Any Star Trek series, regardless of how good, would have suffered from the huge increase in competition for people's entertainment time.