Mandarin is a great language to learn in as much as Japanese,French and German are. Japan has long had a strong economy and exports a large number of goods,same with France and Germany except their colonial past and EU leadership positions makes their languages profitable to learn.
China wants to influence the world and be the dominant super power. however,much like Japan and Korea,they are not interested in expanding their cultural reach. Westerners wanted everyone to dress like them,speak like them and act like them -- the "civilized" way. Sharing your culture eventually leads to immigration and the cultural influence of others impacting yours. Their ambition is purely for the success and prosperity of their own people,not a "manifest destiny" conquest and as such I strongly suspect they have little desire to force others to speak mandarin.
What I'm trying to say is that while Mandarin has good ROI if you learn it,even 50 years from now it will likely still be comparable to learning French,German,Arabic and similar languages,and even then only if you deal with international trade and relations (or read tech/science publications -- seen some great write ups from Korean and Japanese security researchers FYI).
Mandarin should not be thought of a replacement for english but rather a complement. I can see a future where people speak both languages for academia and international relations.