Having needs is not the same as slavery. Wage slavery is described as such because of the power imbalance between employer and worker, who has limited agency to find another employer, with whom he would also have a relationship with a power imbalance.
A wealthy man who receives a check for dividends and interest most months is not subject to such power imbalances. Wealth makes him free.
It's not an argument that socialism would enable people to just live off a public dividend, so to speak. Somebody has to work, and workplaces require discipline. Rather it's an argument for better labor safety controls, and a personal appeal to people to save as much money as they can.
I've been an employer. I didn't have any power over my employees. If there was anything they didn't like, they simply stopped coming to work. How was I supposed to make them show up?
One embezzled a large amount of money and spent it on drugs. What was I supposed to do about that? He was broke, he couldn't pay the money back. All I could do was tell him to not come back.
I've also been employed at minimum wage jobs, and salaried jobs. I never felt the employer had power over me. At my salaried jobs, my coworkers complained about all the power the company had over them. The company had no power over me, so I would ask them what the power was. After some long conversations, the problem was the coworkers spent every dime of their income. So not having a paycheck for a week was a catastrophe. The company, however, was unaware of these issues.
I recommend saving up to 6 months of living expenses, and then the employer will have no leverage over you.