The simplest explanation is that you don't understand the fundamental concepts involved in evolution and therefore are probably not going to get much out of the discussion until you read up a bit.
It would be similar to someone wondering how a rocket engine could work in the vacuum of outer space with nothing to push against. For them to understand, they would have to learn about the physics of conservation of momentum, etc.
Now returning to your question. If monkeys must cease to exist because "man evolved from them" then whatever monkeys came from must also cease to exist, and whatever that pre-monkey came from must have caused the previous species to cease to exist, etc.
Following your logic all the way back -- there should just be humans on earth -- no other organism, plant or animal, since everything has common ancestry.
But that's not how it works. The appearence of a new species does not automatically kill off the old species. That's how we can have many different bird types, for instance, even though they might all have a common bird ancestor.
If you don't understand that, then you don't have a basis for rejecting evolution.
Think of it this way: new breeds of dog are developed all the time. Pit Bulls were developed from other terriers, yet there are still other terriers around. This is because the breeder took a few of the other terriers and bred from them. The other terriers continued to reproduce, and are still around. Or rather, their descendants are: they splintered into other breeds besides.
Likewise with humans: ONE particular group of monkeys developed in such a way as led to humans. All other groups of monkeys developed in different ways.
Please tell me you understand this very basic point now.
6. If humans descended from monkeys, why are there still monkeys?
This surprisingly common argument reflects several levels of ignorance about evolution. The first mistake is that evolution does not teach that humans descended from monkeys; it states that both have a common ancestor.
The deeper error is that this objection is tantamount to asking, "If children descended from adults, why are there still adults?" New species evolve by splintering off from established ones, when populations of organisms become isolated from the main branch of their family and acquire sufficient differences to remain forever distinct. The parent species may survive indefinitely thereafter, or it may become extinct.
That was dealt with in the original post.
The short answer is that a subset of the "ancient monkeys" (primates, actually, our ancestor was neither man nor monkey, exactly) took one evolutionary path (to modern monkeys), and another subset took the path that leads to modern man.
Your question is as nonsensical as if you had asked, "if I descended from my great grandfather, why does my cousin exist?"
It's elementary, first week biology. And yet, ardent creationists ask this year after year, making it clear that they haven't bothered to learn the very first thing about evolution before dismissing it.