Think of it this way: Life always goes uphill, from less developed to more adapted, better at surviving and reproducing.
Early life would be a basic mechanism, trivial in nature, that under the right conditions would kind of replicate itself (kind of, since the replica would be less similar to the original, than when comparing current parent-child similarity). This would basically be a cell. And a cell would remain a cell unless combining with other cells and specializing in functionality would have a certain benefit. Check out this educational video on abiogenesis by cdk007, based on Jack Szostak's work.
But, you know that evolution isn't the process of change of all individuals of a species in a single direction, towards another species. It's a process of forking. So, basic cells of species A could evolve into single cells of species B, but that would only mean the extinction of A if the new environment with the new organisms becomes hostile, if resources are scarce, if species A is less adapted than everyone else, so everyone else occupies A's niche.
Thus, when multicellular organisms evolve, there are A LOT of monocellular organisms already present and occupying important niches. All available sources of energy are being used. If something isn't used - it's a new niche for any accidental mutation that enables the organism to use that niche and prosper as a new species. So anyway you look at it, multicellular organisms won't start existing without single cell organisms surrounding them. So all multicellular organisms will have an environment that includes these "bacteria", so they will adapt to this environment. It's easier to learn to live with something rather than learning to fight with them and then occupy their niche, because the killing and changing to occupy the niche requires much more changes in the organism.
Life on earth is balanced, such as each species occupies its important place in the circulation of substances. This happened not just by chance, but it's the only probable outcome from the process of evolution by natural selection. Having a world that's totally rid of any trace of primitive life forms is next to impossible. On earth we still have species of archaea that are similar to species present on earth billions of years ago. Read this article on Wikipedia about
Nanoarchaeum equitans to get a grasp on the diversity of life.