Many organisms that have successfully survived and evolved have not become more complex. There are many examples, but I will describe only a few... The lowly sponge has lived in the oceans little changed for many centuries. Similarly there are many sea creatures such as the horseshoe crab, sea slugs, clams and plankton that haven't evolved into more complex forms. On land, some plants such as the ferns, moss and lichens have imperceptibly changed in many millennia into more complex forms. Also, many insects and the frogs, turtles, crocodiles and snakes are examples of creatures that have survived but not evolved into more complex forms.
However, some snakes and frogs are actually changing and evolving quite rapidly in an inner arms race that we don't see on the outside. Frogs have evolved toxins that attack the snakes if they eat the frogs, but the snakes evolve to become resistant to the toxins so the frogs evolve to become even more toxic. Is this a form of complexity? Perhaps, but their forms don't outwardly change. They don't develop more complex bodies. They stay basically the same in form although their functions may change, sometimes quite dramatically.
There are many examples of organisms that have survived little changed over the eons, which raises the intriguing question, "Are there some organisms that don't evolve?". Perhaps there are some organisms that don't evolve in form as rapidly as others such as the sea sponges or crocodiles, but they must all adapt to their changing environments. These organisms have successfully adapted, but have had little selection pressures to change their forms into more complex structures.
The other aspect to consider is whether complexity confers a particular survival advantage or not? If something becomes too complex, we can imagine that it may have difficulties in other basic functions such as reproduction or adapting to environmental stresses such as heat or cold. There's an overall critical balance between many competing factors that keeps many organisms from becoming overly complex.
Snakes evolve better, more toxic venoms, while their prey evolve better resistence to those toxins. This is a predator-prey cycle that causes both predator and prey to evolve better molecular offenses or defenses respectively. This produces more powerful and sometimes more complex molecular toxins that are countered by a more efficient molecular machinery to neutralize the toxin in the prey. Are these molecules always more complex? Not always, but they are usually more efficient at killing or immobilizing the prey or toxins, in the case of the prey, to give the surviving predator or prey a selection advantage over those who don't have these more efficient molecules.
This suggests that evolution is itself a somewhat complex and competitive molecular arms race and that the forms that organisms evolve into must be attributed to different selection pressures. Certainly a frog that has an armor-like skin might have an advantage over one that didn't, but this form may severely limit the frogs' ability to function in reproduction or escape, which may lesson this rather narrow advantage. These evolutionary constraints often prevent organisms from evolving into more complex forms without preventing them to adopt to their changing environments with an underlying molecular evolution that enhances their overall survival.