Good old Friedrich Nietzsche had it right when he said 'That which does not kill us makes us stronger'. Or you might prefer Herbert Spencer's 'survival of the fittest'. Both sayings derived from Darwin's concept of 'Natural Selection' – generations either adapt to their environment or become extinct.
Populations exposed to poisons demonstrate the effect well. When poison is applied, the sensitive organisms are killed leaving only survivors who are less vulnerable. Their offspring inherit the resistance. If repeated over many generations, you may end up with creatures that are completely immune to the poison.
Insects are annoying but the effect is already dangerous to humans for another reason. We think we've defeated the common infectious diseases that once kept death rates high – diptheria, typhoid, cholera, typhus, gonorrhea, syphilis, tetanus, leprosy, pnuemonia, tuberculosis etc. Actually, the bacteria that cause these diseases are only being kept at bay by antibiotics. Unfortunately all these bugs have developed antibiotic resistance and we are running out of ways to control them. The type of Pencillin deadly to bugs in 1945 is now ineffective. Unless someone comes up with something new most of the old epidemic diseases will be back. Quite likely your local hospital is already struggling with outbreaks of MRSA, streptococcus, C Difficile and others.
Another major problem with powerful insecticides is that Sod's Law applies – you kill all the wanted insects like Ladybirds and Bees and end up surrounded by super-blowflies.
Dave