It was clearly demonstrated later in the movie that the habitat could survive Martian dust storms. Why was the crew in such a rush to leave?
The pilot was worried that if the MAV (Mars Ascent Vehicle) tipped beyond 12 degrees, it would topple over and that they would be stranded on the planet's surface with no means to re-enter orbit. When the speed of the storm was announced (e.g. sufficient to knock over the MAV), the Captain took the decision to scrub the mission and fall back to space.
In the event, the storm hit slightly earlier and with greater force than expected and the module tipped to 13 degrees. The pilot used a reaction thruster to temporarily upright the escape module, but this was only a short-term solution that cost them fuel and bought them a few extra seconds.
To answer your specific question, had they decamped to the HAB(itat), they would have all died of starvation within around 70 days when the food supplies ran out.
The description in the book is quite similar;
“Strap in!” he yelled to the crew. “We’re tipping!”
The MAV creaked as it tilted faster and faster.
“Thirteen degrees,” Johanssen called out from her couch.
Buckling his restraints, Vogel said, “We are far past balance. We will not rock back.”
“We can’t leave her!” Beck yelled. “Let it tip, we’ll fix it!”
“Thirty-two metric tons including fuel,” Martinez said, his hands flying over the controls. “If it hits the ground, it’ll do structural damage to the tanks, frame, and probably the second-stage engine. We’d never be able to fix it.”
“I’ve got one trick. If that doesn’t work, I’m following her orders.” Bringing the orbital maneuvering system online, he fired a sustained burn from the nose cone array. The small thrusters fought against the lumbering mass of the slowly tilting spacecraft.
“You are firing the OMS?” Vogel asked.
“I don’t know if it’ll work. We’re not tipping very fast,” Martinez said. “I think it’s slowing down…”
“The aerodynamic caps will have automatically ejected,” Vogel said. “It will be a bumpy ascent with three holes in the side of the ship.”
“Thanks for the tip,” Martinez said, maintaining the burn and watching the tilt readout. “C’mon…“
“Still thirteen degrees,” Johanssen reported.
“What’s going on up there?” Lewis radioed. “You went quiet. Respond.”
“Stand by,” Martinez replied.
“Twelve point nine degrees,” Johanssen said.
“It is working,” Vogel said.
“For now,” Martinez said. “I don’t know if maneuvering fuel will last.”
Actually, they left for no reason whatsoever. The dust storm was never strong enough to push over the MAV, nor was it strong enough to nearly kill Watney with the antenna. It was just a plot element, not something that is scientifically accurate. So use whatever excuse you want, it just sets up the fun of the story.
The dust storm, according to the book, blew over 150kph. Very fast, for Mars. However, the surface pressure is only about 7hPa,or 0.6% of Earth's surface pressure. Even assuming wind pressure force scaled linearly with pressure [it doesn't] the force on the MAV was about what you'd feel with a 0.5 or 1/2 mph wind on earth. Even being extremely generous with the math for the weight of the dust, you're still looking at under 5mph earth equivalent. Gravity is weaker, but not nearly weak enough for that to be a issue.