Organisms do not always evolve into more complex forms.
The alligator and the shark are wonderful examples. These creatures have remained virtually unchanged for eons.
The flu virus has evolved forever, changing from one kind of flu virus to another. It has not gained complexity, it has simply changed.
Algae has not changed over millenia. Ditto for paramecia.
Other organisms have become more complex... but that is because environmental challenges have made the more complex creatures better suited to new environments. For example, a longer-legged chimp had better survival prospects than a short-legged one when rain forest became savanna. So the longer-legged chimp had more children, and that strain prevailed.
If an organism is well-suited to surviving in a variety of environmental conditions, it could easily reach the level of complexity needed to survive, and stop there. There's no reason to become more complex.
You've heard the old story about what would survive a nuclear holocaust. It would be the roaches. But it's the truth... they are the ultimate omnivores, they have not evolved for ages, and will be with the planet long after humans are gone. They've reached the level of complexity needed to survive, and there is no reason to evolve further.
If a roach was born with ten legs, would it survive as a new species? Not unless it thrived and had children with ten legs who also thrived. And they in turn bore another generation. But if the trait was not passed on, or if the progeny did not thrive? Then the ten-legged roach would disappear.
The notion that evolution leads to increasing complexity is ridiculous. Evolution leads only to improved relations with the environment. When an environmental challenge... like the lack of food, or changing water levels... puts a species under stress, some members of that species will survive. They in turn will have children, and the children will often possess the traits that are needed for survival.
And if the children do not possess those traits? Then they die. End of species.
Evolution does not necessarily lead to more complex organisms, it simply leads to organisms that adapt better. But a creature like the shark, which is so perfectly adapted... needs not adapt.