Easier said than done. What are they going to retrain to? As AI continues to advance, more and more jobs will necome redundant. CGP Grey did a great video on this[1]. Even if you can't completely replace humans, often times, obe person will be able to do the job that used to take several. Where do those others go? What will we as a society do if for example AI-based Expert Systems make 90% of accountants and office clerks redundant? We can't retrain all of them to become doctors and scientists, can we?

[1] http://youtu.be/7Pq-S557XQU