Imagine that all microorganisms suddenly appeared from the earth, why do you thi

I'll try to answer:

Firstly, most animals and plants needs microorganisms to live. For example, at humans some bacteria produce essential vitamis. Humans could not live without bacteria in their body.

But lets assume that some animals and plants could live without microorganisms in their body.
Microorganisms (specificly decomposers) are also very important because they "recycle" organic matter and brake it down into simpler forms of matter, which are essential by producers. (Some kind of Worms are also considered decomposers, but in fact, some bacteria inside the warm decompose the organic matter.) So, without microorganisms, the producers would eventually ran out of the inorganic matter they need, and die. In that case, the rest of the higher-organisms would also die.

Microorganisms could survive without higher-organisms present.
Firstly, there are producers microorganisms, which would provide energy to the microorganism ecosystem. Their role would be similar with the role of the plants for our ecosystem.
Secondly, there will be the decomposers who would continue to decompose organic matter providing the producers the inorganic matter they need.
These microorganisms would be enough for the ecosystem to survive.
Some other microorganisms (consumers) would also survive (acquiring energy that is provided to the ecosystem by producers).

1. Because microorganisms are much more adaptable to sudden changes.
2. Because there are trillions of them per one of us, so there are often variants that survive sudden changes.
3. Because for most of history, they lived without "higher organisms"