Unfortunately, they strayed too far from their original premise, and lost their audience. Moving into the direction of a sci-fi sitcom alienated the original fans of the show, but failed to appeal to a sci-fi audience.
During the hiatus between seasons 5 and 6, an off screen alien invasion of Earth occurred. Charlene and Suzanne died during the invasion. The remaining characters were abducted, and taken aboard an alien ship. The aliens had also abducted Julia's cousin Alison, and while sorting their prisoners discovered the relationship via DNA analysis. Alison was therefore housed with the Sugarbaker group.
The ship was taking the Sugarbaker group to their home planet of Omicron Persei 8 when a catastrophic failure occurred in their FTL drive during a jump. The ship emerged from hyperspace well short of its destination, however, to everyone's surprise, the ship emerged into normal space traveling at approximately .9C - a feat previously thought impossible.
To keep people occupied during the remainder of the flight to OP8, the aliens engaged the Sugarbaker Design firm to redecorate the ship as it made its way back to the home planet. The cramped conditions aboard the ship, and the long travel time caused friction, and Alison did not prove to be a good fit with the rest of the firm, and was jettisoned via an airlock in the general vicinity of the Horsehead Nebula. This occurred in the season 6 finale.
A new character, BJ was introduced in season 7. She was not related to any of the characters, however the aliens thought that teaming her up with Julia would create interesting interactions for their amusement. While BJ was better received by the firm than Alison, the aliens began to lose interest in the project.
Realizing that the life span of humans was too short, and all humans would be dead before reaching Omicron Persei 8, which was estimated to take at least one thousand solar years, even traveling at .9C, the aliens decided to put everyone in hyper sleep (cryogenic suspension) for the remainder of the journey.
In the final episode, after being placed in hyper sleep, the Sugarbaker team dreams that they are in the old south in a parody of Gone With the Wind, set in Julia's old house back on Earth.
So there you go, OP. That is the story of the decline and fall of Designing Women.