Real life is scarier than you think, as in the past their have been corporate entities that did have their own private armies and ran countries like they owned it. The East India Trading Company is a great example as well as the way Britain acquired a lot of their empire in the 19th century through economical takeover rather than actual military means.

The rise of PMC activity in the last few decades also means it is plausible for a company like Google for instance to hire a private army if they wanted to, so the whole Shin-Ra thing has not only happened before but is possible to happen again if people don't really pay attention. Whether they could become a global super power on a military scale is largely unlikely but some people would argue that corporate interest in politics has largely made countries like the U.S. and Great Britain into slaves of things like Big Oil.