I do not agree. Chinese Mandarin is incredibly easy to learn, should you ever try to get interested to it. There's no verb, no gender, no plural form, no proper noun (as we understand them) and its grammar is probably the simplest of all languages of mankind.On the other hand, Mandarin is too hard to learn and Hindi barely reaches into the diaspora.
What's difficult, however, is to pronounce it properly. But then again, it's not that varied once you catch the "accents".
Really, it's a serious contender. More than you think.
Hindustani, on the other hand, is quite the opposite: it's a very difficult language full of exceptions and specific rules. Even if it has been greatly simplified.
However, you have to add several other dialects which are very close to Hindi, like Urdu for instance (spoken by another 200 million speakers).
And so on. So with the growing industry of Indian medias that prefer to use Hindustani rather than English (it's a new trend), things might change sooner than we expect. And once again, remember that English is not understood by 93% of current Indian population. And their total number are not increasing, even if 7% of 1.150.000 people is still a lot.
History is not written.
French probably is too precise as a language. It's perfect for diplomacy, but not as a global lingua franca.French on the other hand, that perfection of Latin, has been our lingua franca for a millennium. If only French, the language of diplomacy, would grace our lips once again, we would, once again, be graceful.
But in the absence of grace, we shall be barked at by an English mongrel.
In comparison, English looks a lot like Esperanto, and is far easier to learn, one of the critical reasons of its success.