Since I can't comment on you're taste selections, I'll do it here.
Gillespie says this:
"There's no simple accounting for the decline in English and foreign language enrollments. (Indeed, there's no simple accounting for their increase in the period from the end of World War II through the 1960s)."
I don't believe him when he's says there's no accounting for it; the 60s and 70s were times when our society was really at the top of the heap economically and college students were experimenting with majors that wouldn't necessarily lead to a firm financial future. They were willing to try more besides the drugs we associate the period with. And in the 80s and 90s we saw more conservative choices in majors, those going into business and economics and medicine because those were, as far as they collegians were concerned, stable professions in what we can say were economically dismal decades, discluding the later 90s.
I'm regurgitating this from somewhere. I'll see if I can find it.
I buy the experimentation angle but I'm sympathetic to the view that there's no easy accounting for these enrollments.
As far as the economy goes, however, what you're saying may just sound familiar because it's such a commonly-held belief. It's probably somewhat true that the economy influences our choice of majors, but I don't know if history supports it as a major/simple cause. I wouldn't lump the stagflated 70s in with the flush 60s, for one thing.
And are you suggesting that there was a corresponding rise in humanities and foreign language majors in the later '90s? I don't know if there was, but it would be interesting to check.
There very well could have been. Gillespie does say that there was a slight resurgence in the disciplines in the last few years that would coincide the economic upsurge of the late 90s.
We are ill-advised really to do any theorizing about economies and their effects on education. We did pick English over business and economics, after all. Like you mentioned, it's a commonly-held belief from which I'm theorizing, not statistics or facts I could possibly reference had I chosen business.