Who pays for these job guarantees, and how do we ensure that there are meaningful jobs always available that fit both a persons skills and interests? Or are we assuming that anyone unable to find a job would jump at the chance to do whatever scrap job the government is willing to throw their way?

Markets are important in the system we've built, we can't just toss out market effects and assume nothing else will break. Whether markets are the "right" approach or the one that a majority of people still want is up for debate, but in the meantime much of our society is based on markets operating relatively freely as the approach of both understanding sentiment and allowing markets to shift without centralized control. Getting rid of the downside in our job market would almost certainly break the job market all together, as there are few downsides motivating people to try to find and keep a job.