You slightly misunderstood the concept of majority here. Bitcoin relies on the majority of trusted computational power. So it does not matter if there will be billions of small clients controlled by the same owner if they are not powerful enough.
The main idea in the beginning was that each client is running independently. So if there will be a lot of independent clients, then for any person who would try to harm the system by combining a lot of computational power (and thus manipulating the results) it would be really hard, because he would be outnumbered by a trusted majority.
But with the idea of mining pools a lot of things has changed. If you will look here: you will see that 3 major mining pools control more then 51% of hashrate.