The only smart investor is a contrarian investor.
No matter how hard you try at Spanish, you will never be as good as a native speaker, and you will have to compete against multitudes with the same qualification. The fact that there are so many more people who speak Spanish has no bearing on one's personal gain from being able to speak it as well. Greater gains are to be had doing that which very few people can do at all, let alone well.
In any setting where Spanish is the dominant language, the means are typically abundant to get by with English. You mention Hispanics are the fastest growing demographic. Yes, and just like every other American immigrant demographic that has seen rapid growth, as they become more numerous they will become more diffuse, and their mother tongue will become less useful as they assimilate into American society. What makes you think Spanish will be any more useful in 20 years than Yiddish or Gaelic or Polish?
Because French is niche, and the people who speak it are not as integrated with American society, there are many more situations where the language has irreplaceable utility. There are many fields where French is still the dominant language, including food and wine, foreign diplomacy, and classical culture. And many former colonies on all continents still speak French.
France is bent on making French the dominant language of the EU (really, imagine that), a rising power whose influence will expand at the expense of the USA and PRC as they fall prey to their own intractable structural problems. Even now, the EU is wealthier than the US, and being able to speak French is an enormous advantage in dealing with them and taking what they have to offer, far more lucrative than the hardscrabble Spanish-speaking world.
More opportunities are available for Spanish speakers. But more lucrative, and less competitive, opportunities are available for French speakers.
In conclusion:
