Don’t worry too much about someone’s predictions of the future; they’re almost always wrong. At least, partly wrong. Tenure is an ancient tradition, and it probably won’t go away entirely; even if it becomes less common, it will probably remain possible, and universities will continue to have professors at various ranks, including tenured full professors. The $20k/year salary is ridiculously small. I’m not sure what your source was, but it’s just not possible – universities couldn’t keep a faculty if they paid that little.
You’re certainly right that it takes a lot of work – four to six years, generally, of graduate school depending on your college and field. (I’ve known PhD candidates who’d been in the program for even longer, but not many of them.) After that, it’s necessary to spend several more years doing postdoctoral work. Then you get to be a junior lecturer (or the equivalent).
My guess is that math and compsci are safe choices (though my plans are in chemistry, so I have similar hopes, and there’s always a chance I’m horribly wrong). Supposing the direst predictions about professorship come true, you’d still stand a strong chance of finding a high-paying position outside academia.
Anyway, most of what I’ve read about the future of post-secondary teaching has been more positive than what you mentioned. A large number of tenured professors are reaching retirement age, and many replacements will be needed within the next 10 years. (I’m not sure how well this applies to compsci.) Enrolment rates are generally increasing, so the demand will probably continue. Note that many fields – math and compsci included, I think – essentially require a post-graduate degree for employment in industry.
One other thing – I’m not sure how far you are in your studies, but it’s my opinion that double majors are not a good idea. Computer science requires a lot of math, so you might find you qualify for a math major or minor, but it’s highly unlikely if not impossible that you could pursue both math and compsci at the graduate level. It’s far more likely you’ll end up specializing in an area of one of the two fields that interests you. Your choice of graduate supervisor will probably determine your career path.
Do not heed the predictions of ‘what’s hot and what’s not’ in higher education. What is a trendy degree now will be an oversaturated job market when you graduate. I mean, don’t study an area of your field that some ‘experts’ are calling the next big thing… study what you’re interested in and what you do well. You don’t want to study something you’re not absolutely passionate about for six or eight years just because someone else told you it’d be profitable.