I took 5 years of French in public school, some 30 years ago. The benefit was almost nil. All the schools in those days, like now, taught French and Spanish. I've never noted our business or cultural relationship with France or Spain (or other French or Spanish speaking countries) doing anything special (vs., say, Germany or Italy or Japan) as a result. And small wonder, since my French education enables me to read a menu and perhaps the odd museum sign in Paris.
Going back many many years, written and spoken Latin and ancient Greek were required studies for the educated person. Few take Latin and just about none Greek - in fact we mostly find the whole idea quaint (though I much enjoyed my one year of Latin). I look back on mandatory French and Spanish and think the same - it did just about nothing for us. Additional science, math, history, world culture, economics, art, etc. - just about any of them would have been time better spent for the vast majority of kids.
Many proponents of Mandarin Immersion - AND broader elementary language instruction (FLES) - hold out the growing importance of China in business as a key reason to learn Mandarin. But is it true? I have read that in a decade or so, China will have more English speakers than the US, and that English instruction is now mandatory in China from 3rd grade on. So many Chinese, esp those dealing with foreigners, will speak English (as do many, many foreign-facing Europeans). And that's not because the US is important - it's because English is the lingua-franca of business worldwide. So how useful will our efforts really be?
Wouldn't it be better to see our reasources - dollars, classroom hours, adminstrative attention - go to things we KNOW are and will be important - math and science, writing and reading, history and world culture. Understanding Chinese history, culture, and politics seems FAR more useful to me than trying to learn Mandarin (either by immerision or otherwise). I don't have to speak French to fathom how the French behave or understand the drivers of French business and politics.
So let the flavor-of-the-month come and go - be it MI or FLES. Let's beef up our basics, esp in K-5, and make sure our kids are prepared for whatever the world serves up in 30 years.