You can top dress the lawn every year and get super results by adding compost or composted manures. I get compost by the truck load or dump truck load and spread it early in the Spring. After the lawn gets healthy you can skip every other year. In about 5 years you should have nice lawn.
The composts and composted manures are loaded with Billions and Billions of soil microbes! You have a over a million microbes just in a teaspoon if soil. All nutrients have to be broken down by microbes first, before the plants and use the nutrients. Without microbes the plants couldn't live. The composts don't have that many nutrients, but the "microbes" are the main reason to use composts. Then, when you broadcast your lawn fertilizer(21-0-0) the microbes break down the nitrogen fast so the lawn can turn thick, green and healthy. The microbes breakdown the thatch in the lawn and never needs dethatching.