50 years from now, Mandarin will be much more useful than Arabic or French, because it will be the largest market in the world. If you deal with China professionally, knowing Mandarin will be an advantage.
Sure, as a regular boring American 9-5er you won't need it but if you want something more from life you better learn a language and Mandarin is objectively going to be the most useful.
Not as many stories about people jumping out of factory windows to end their lives in Africa. There are many hidden gems in Africa as well with perfect climates and cultures that have little to no thievery or need for distrust. These things aren't well-known or stereotypical because Africa is dismissed outright so quickly by so many without further investigation, but it's true anyway.
In 50 years, I think this will remain true and Africa becomes even more desirable yet than China. If you want to work, or want your less fortunate children to work at Foxconn pumping out iPhones with management practices that westerners cannot even imagine at this point in their history, China may be an option if they'd even welcome you. Of course everyone assumes they'll be the boss at the desk browsing HN or scrolling through Facebook during the workday instead, generally enjoying the good life.
Africa. This isn't even a contest.
I'm not sure what you mean by this. It's the language with the most speakers, maybe? It's the one you think would give the greatest money-making advantage? There are many other ways you could decide which is most useful, e.g. language spoken in the most countries, but 'objectively most useful' depends on a lot of unmentioned things (Useful for what?) and seems a fairly meaningless phrase to me.
Of course, many countries are encouraging English learning, and I think a lot of former French colonies are interested in moving towards English, but after English (59 countries), there is French (29), Arabic (26), Spanish (20), and Portuguese (9). Some of my best friends are "Arabic" speakers, and they're from the heart of the land (Saudi Arabia) and tell me there is not a single "Arabic" language that all nations can entirely mutually understand, so I'm convinced of the validity of that one entirely. Spanish and Portuguese nations definitely have mutual intelligibility and seem very steadfast in maintaining their languages.
I would agree with your general point for sure that an unflinching statement like "Mandarin is objectively going to be the most useful" is more of a flimsy limb, given more information and taking looks from additional perspectives.
If I had to bet my life on a language being the "most useful" going forward, I'm putting my hide on English. I think if Mandarin being most useful were true, it would already be an unquestionable statement today. Not sure how an already mostly-risen power like China has one of it's two languages (and not the one spoken in Hong Kong) become "more useful" tomorrow than it is today, and it's not an especially useful language right now outside of China. I wouldn't even venture to say it's #2 by many metrics.
China and it's official language, Mandarin, have a lot of hype about domination and influence, considering it's a relatively insular culture. Flat statements on the Mandarin topic are tough to swallow. My guess is outside of government interactions, Chinese business will place Chinese management and even Chinese workers in their enterprises, if at all possible[0]. They won't need John Doe to speak Mandarin, nor will that person really even be desirable. Those people are learning English and they prefer their own.
[0]https://www.commondreams.org/news/2018/11/06/absolute-fraud-...
[0]https://www.livescience.com/40247-ashkenazi-jews-have-europe...
(Except for your own benefit.)