You're messing up comp sci. with industry programming trends. A comp sci phd is about exploring different fields of computer science or using cs skills in a new applied way; even if the language isn't used much anymore, you'll still retain all those skills and be able to put them to good use in some field. If your phd expired, it wasn't worth much in the first place. I'll give you an example: A lot of my research is basically number crunching on vast swaths of data. I mostly use python, which is the cool trendy language, but if python goes out of fashion or the company uses a different language, I can still do all my number crunching 'cause I know the math and I know how to learn new languages. The language/software/etc. is basically just a tool to get the research done, it shouldn't define the research. This is even true of thesis that involve new langauge design, 'cause the language is always just the prototype of whatever ideas the person was studying.
Depends on the field. Some jobs require a phd, others lots of work experience; which jobs you get will be as much a function of what you apply for as anything else.
Can you code? I don't think a phd in management is gonna help you in any way shape or form unless you want to be a management consultant.