The Bitcoin eligible voters are not "the majority of computing power in existence" because computing power is not a fungible, homogeneous substance. You can easily see a 10^4 performance ratio on specialized versus commodity hardware (ASIC vs CPU), so that the Bitcoin network becomes impervious to attack if it makes up only 0.01% of the "computing power of the world" as expressed in transistors*Hz. Rather, Bitcoin, like most other currencies  in the world, is up against any adversary more financially powerful than it's backers (the miners). So if you are willing to invest more than the compounded mining profit, you can take the majority vote and influence consensus, by expanding the computing power of the world in the form of efficient mining machines.

It's pretty clear that rewriting the history is not equivalent with stealing everybody's money, rather it means destroying the system and making the coins worthless, so the likely attackers will not be profit-motivated by any definition of profit expressed in bitcoins. We could talk about governments, banks, competing currencies, lulz etc. It's only a matter of speculation if an attacker likely to act in such a manner exists. Furthermore, as the network expands the window of opportunity closes to exclude small scale lulz-motivated attackers, and allow only governments or large corporations. The hashing power of the network already surpasses what could be accomplished by ~10 million commodity PCs, excluding even the largest botnets as worthy attackers.