The abyss is the netherworld the prison of spirit powers. The demons did not want to be sent back to the abyss, either because they knew it was a prison for them, or because they knew that even there, Christ's power was effective. So the pigs stampeding down the cliff and into the sea was no escape from Christ's realm either. The point Luke is trying to make here is that Jesus has moved out of the territory of his own people to be among the Gentiles or non-Jews. The demons are not able to withstand those who preach, teach, and heal in the name of Jesus who sent them.
Evil is as real and present today as it was then.
People still suffer from disease physical and mental. Great amounts of money are spent on military and police forces and equipment in countries, where they do not keep simply keep peace or enforce law within the law but brutalize the people into submission to dictators. Environmental practices in many places make you wonder if this planet is going to survive. Children, women, senior citizens and some men, live in fear from violence and abuse, drugs and alcohol, gambling addictions, poverty and other destructive strains on the family. Bureaucracies that grind out results, rather than justice, scam artists and thieves, liars and cheats still plague our world and the list could go on ad infinitum.