Wrong. Apes are a super successful group and pre-grass they made a lot more sense. There were major risks to going the walking ape route.
1. More predators out in the open on the plains. Especially big cats.
2. Overheating was a big problem. Human have to be on the move and go all over for food instead of snacking on jungle plants all day. Humans had to develope both hairlessness, sweat glands, and upright legs to deal with the lifestyle.
3. Is the probably the biggest. Upright walking means that the birth canal is tighter than any other animal. Infant mortality is the biggest barrier to it that you can imagine. It means humans have to be born many months premature in order to even fit through. Infant mortality and even more importantly maternal mortality is very high for humans in the wild compared to other animals. The premature birth also means babies need looking after longer and thus mother's cannot reproduce as fast either. Chimp babies out pace humans until the are 1.5 years or so I think. They walk faster, they can climb and escape predators and they are mentally superior because they come out more fully developed because their skills fit more easily through the bird canal.
4. The human brain require so much fuel, it would be incredibly difficult for humans to get enough nourishment on the diet of a chimp or gorilla.
So really Humans are at a huge disadvantage in so many ways, but the human brain is the most amazing advantage as is human culture, that all those drawbacks are worth it.
Lastly, we are
not the only ones who have done it. There have been dozens of humanoids as you call them, or hominids, that have died out. Now most believe they didn't die out for obscure reasons. Neanderthals were successful for 100k years, longer than we have been around. Maybe we killed them.
Pretty easy read
http://discovermagazine.com/2011/evolution/23-how-we-won-the-hominid-wars