Many of the shows that you complain had to compete against each other got on the air because TNG proved that adult sci-fi was a marketable commodity. And as far as I know, DS9 and Voyager never had to compete head-to-head for viewers. One aired mostly on the weekends in syndication while the other was on a network (UPN), Wednesday nights at 8pm.
It did here in New York.
DS9 and Voy. ran at the same day and time for a few years.
Eventual they switched Voy. to a new night.