But the boys don't mature!! That's not writing a writing flaw--that's a flaw into the boys that the writers built in!
It's kind of funny how many things we didn't want to happen actually happened. Particularily Stevie being sort of cured for an episode, Lois being pregnant (that one test could have been a false positive, you know), Dewey having a girlfriend (who said he would have a boyfriend!?), and Malcolm remaining a total social reject. Nonetheless, we all agreed it was an awesome season. Which goes to show you how truly talented the MITM writers are.