There are infinately many types of factory jobs, as varied as the companies that support them. While some types of jobs are being taken over by robots and/or computerized control, there are still many things that need to be done, that for one reason or another cannot be done by a machine. Examples are system maintenance and repair, integration, and yes, janitorial work. Also, quite simply, humans are typically cheaper labor. I'm sorry, but there are plenty of college students, single mothers/fathers, and new immigrants (legal and illegal) to compete directly with technology for efficiency.
Very technically, I am a factory worker. Most of the companies I work for are industrial, and thus I get to see inside many different styles of factory environments. One such company packages the casing for hotdogs (the stuff that it is cooked inside, not what you eat). I would estimate there being somewhere around 20 "menial" workers, who operate the machines and take care of the rolled casing. While I have no idea how much these people make, I know they are not "rotting away" as they perform their somewhat-limited functions. Do they make as much as I do, the hotshot contract worker with the fancy laptop and technical know-how? Doubtful. While I wouldn't call myself highly trained, I have worked very hard to get where I am. That's the point of capitalism.
The problem with replacing "95% of factory jobs with robots" is that, quite simply, robots aren't that good. I deal specifically with this task of creating solutions that will automate an assembly, eliminating the human element as much as possible. The cost of developing a simple part-sorting machine is well above several years of a single worker doing the same task. The machine would be able to do this much faster, but the machine would also require several people to build it, operate it, and maintain it. The more complex machines, such as the automation for car assembly plants, requre small armies of humans to do these jobs. In fact, my father is one of these said "soldiers", at a new GM plant in Lansing, MI.
That said, any type of job can become menial and slave-like with the mere application of one's imagination. If a job is intolerable, one should make changes, whether by changing the nature of the job or by leaving the job and finding something more to one's liking. Otherwise, there must be something "ok" enough about it to make one stay. While the "capitalist pig owners" or (sinister voice) "The Corporation" may or may not be making a killing because of these employees, these days most companies will reward the people who help them make that money, including their wage slaves.
Afterthought: most of my factory experience is in the Chicago-suburb area. I mean no offense to 723, now that I have read that he (seems to have) worked in the Rockford area. Companies and corporations are just as likely to abuse its workers as reward them. Human nature, I suppose.