Regarding the first point, observe that the number of people doing mathematics majors does increase over the time period so that wouldn’t be an explanation for the lack of growth.
Regarding relevance to jobs, I think CS is, de facto, the major that employers see as more relevant in candidates in the US, though I think this is much less about the content and more about CS being seen as more prestigious and selecting for more ambitious/capable students who may make better employees in the long term. I don’t think the reason for the relative decline in the popularity of CS was that students really wanted to do software engineering (but ended up doing physics/math instead).
Regarding relevance to jobs, I think CS is, de facto, the major that employers see as more relevant in candidates in the US, though I think this is much less about the content and more about CS being seen as more prestigious and selecting for more ambitious/capable students who may make better employees in the long term. I don’t think the reason for the relative decline in the popularity of CS was that students really wanted to do software engineering (but ended up doing physics/math instead).